Allium shortbread
This is part of our series on upcycling farm by-products and fruit & veg by-products.
Table of Contents
i. Background
Alliums—onions, garlic, leeks and more—are some of the true staple vegetables, unsung heroes that aren’t always noticed when they’re there, but certainly are when they’re not. Many cuisines use an aromatic combination of vegetables as a base for many dishes, which almost always contain at least one allium—think French ‘mirepoix’, Italian ‘soffrito’, Cajun ‘Holy Trinity’ and many more.¹
Onions are particularly prominent, and are the world’s second most-produced vegetable after tomatoes, with 111 million tonnes produced globally in 2022.² Yet they are so very undervalued. Few vegetables are sold at a lower price in supermarkets, and many alliums and their by-products are wasted in abundance on farms, during commercial processing, and in both restaurants and home kitchens. Yet all these alliums possess immense untapped flavour potential.
We present two different flavour-packed powders that demonstrate that potential, one made with onion trimmings and one made with leek trimmings, which we then use to make variations on a savoury shortbread. The onion ash has a complex, smokey umami flavour, whilst the leek ‘matcha’ powder has a lovely sweetness, umami flavour and bright colour surprisingly reminiscent of matcha—hence the name.
Onion and garlic powder, already familiar to many cooks, are usually made of the flesh of lower-grade alliums. These techniques offer further ways of getting similar flavours from allium trimmings that would otherwise be wasted.
ii. Recipe
a. Onion ash
Ingredients
Onion trimmings e.g. skins and any outer onion layers which may have been removed during peeling, and top and bottom trimmings, cleaned
Method
Dry the onion trimmings in a dehydrator or an oven at 60°C until they no longer contain any moisture.
In a deep hotel pan or metal pot, light the onion skins on fire. Allow them to burn three-quarters of the way and then cover them to extinguish the flame.
Grind the smoked trimmings into a powder in a spice mill or high-speed blender.
b. Smokey onion shortbread
Method
Add the softened butter to a stand mixer with a paddle attachment and mix until fluffy and slightly lighter in colour. Add in the cheese and onion ash, and mix for 2 minutes or until combined and fluffy. Add in the flour and mix until just combined.
Form the dough into logs about 3cm in diameter and chill for at least one hour.
Slice the dough into discs about 1cm thick and lay them on a baking sheet. Bake at 160°C on the fan setting for approximately 12 minutes.
Dust on some of the remaining onion ash to garnish, if desired.
Ingredients
100g salted butter, room temperature
100g parmesan cheese
25g onion ash (recipe a)
100g flour
c. Leek ‘matcha’ powder
Method
Dry the leek trimmings in a dehydrator or an oven on its lowest setting until they no longer contain any moisture.
Grind the dehydrated trimmings into a powder in a spice mill or high-speed blender.
Ingredients
Leek trimmings, tops and bottoms, cleaned
d. Leek ‘matcha’ shortbread
Method
Add softened butter to a stand mixer with a paddle attachment and mix until fluffy and slightly lighter in colour. Add in the egg yolk, and mix to combine. Add the leek ‘matcha’ powder and mix for 2 minutes or until combined and fluffy. Add in the flour and mix until just combined.
Form the dough into logs about 3cm in diameter and chill for at least one hour.
Slice the dough into discs about 1cm thick and lay them on a baking sheet. Bake at 160°C on the fan setting for approximately 12 minutes.
Dust on some of the remaining leek ‘matcha’ powder to garnish, if desired.
Ingredients
100g butter
20g egg yolk (or 20g egg white)
25g leek ‘matcha’ powder (recipe c)
5g salt
5g sugar
100g flour
iii. Adaptations
This recipe might also work well with trimmings from garlic, spring onions and other alliums, or a mixture, either burned or not. We’d encourage you to try different combinations and techniques, though not all work as well. For example, we didn’t like the onion powder so much if it was only dehydrated; burning the trimmings removed raw off-flavours, which might also work if trimmings are roasted instead. Trimmings of alliums can be dehydrated and stored for convenience, before processing them in bulk.
We liked the depth that the cheese added to the shortbread recipe, but you could leave it out so you get more of the allium flavour, or you could also try substituting all the animal products with plant-based alternatives if you desire, though in both cases you may need to tweak the recipe to achieve the right texture.
These powders can be used for much more than shortbread. You could use them in much the same way as you’d used conventional onion or garlic powder, adding umami depth to dishes like stocks, stews and sauces. These also make great garnishes: the leek ‘matcha’ is especially nice as a garnish that lends a pop of colour.
If you make these allium powders, adapt the shortbread recipes or use them for something else, we’d love to know.
Contributions & acknowledgements
Kim performed the original culinary R&D. Aly conducted further R&D on Kim’s recipe, and documented the process with additional notes which Eliot used to help write the article following further discussions with Kim. Josh contributed editorial feedback. Eliot photographed the onion trimmings and Aly photographed the final products in our food lab.
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Endnotes
[1] Lindsay Howald Patton (2020) ‘All About Mirepoix, Sofrito, Battuto, and Other Humble Beginnings’, Serious Eats.
[2] FAO (2023) ‘Agricultural production statistics, 2000–2022’. Yes, tomatoes are botanically classified as a fruit; here the FAO seems to have a more culinary classification in mind.