Step 1

Prepare the sour onions 1 month ahead of when you plan to make the canapés. Place the Roscoff onion quarters into a jar. Mix the salt into the whey and pour over the onions, making sure they are completely covered. Seal the jar and store in a dark place for a month to ferment

Step 2

A week before serving, make the nasturtium capers. Prepare a brine by adding the salt to 300ml of water and bringing to the boil in a small pan. Remove from the heat and pour a third of the brine over the nasturtium buds, leaving to soak for 24 hours. Reserve the remainder of the brine

Step 3

After 24 hours, drain the buds and rinse with cold water. Pour another third of the brine over the buds and soak for another 24 hours. Rinse and soak for a final 24 hours with the last of the brine

Step 4

After this third soak, make a pickling liquor by adding 100ml of water, the white wine vinegar and caster sugar to a small pan and bringing to the boil. Once the sugar has dissolved, pour the hot liquid over the buds and leave for at least 3 days to pickle (these will keep for up to a year in the liquor)

Step 5

On the day of serving, make the oyster emulsion. Add the sliced shallots to a saucepan along with the white wine. Place over a medium-low heat, boil until all of the wine has evaporated then remove from the heat and allow to cool

Step 6

Once cool, add the shallots and oysters to a blender and blitz until smooth. Gradually add the oil and continue to process until it comes together in a mayonnaise-like emulsion. Add a little of the reserved oyster juices to loosen the mixture and chill until serving

Step 7

For the shallot crisps, add the butter to a wide shallow pan and melt over a high heat. Once the butter starts to foam, stir in the sliced shallots and cook until golden brown and slightly crisp. The the butter should smell like roasted nuts and be a deep golden brown

Step 8

Remove from the heat and strain the shallots through a sieve. Spread out the crispy slices on kitchen paper to drain any excess fat and sprinkle with a little salt. Once dried, finely chop the shallots

Step 9

For the beef tartare, place the chopped beef into a bowl with the mustard, oil and 1 tablespoon of the pickling liquor from the nasturtium capers. Season to taste with salt and pepper and thoroughly mix the ingredients

Step 10

To serve, remove the sour onions from the whey and separate out the layers to form little shells. Spoon a little of the oyster emulsion into each onion shell, followed by some of the beef tartare mixture

Step 11

Garnish with a few of the nasturtium capers (drained from the pickle), the shallot crisps and finally some fresh nasturtium leaves