How to Make It
Make the pickled onions Pack the onion petals into a clean 2-cup glass jar. In a small saucepan, combine the lemon verbena with the vinegar, sugar, 1 teaspoon of salt and 1/4 cup of water; bring to a boil. Pour the brine over the onions; add additional warm water if the onions are not fully submerged. Close the jar and refrigerate the onions overnight.
Make the lemon curd In a small saucepan, whisk the lemon juice with the egg, corn syrup and salt. Bring to a simmer over moderately low heat, whisking constantly, about 3 minutes. As soon as the mixture comes to a simmer, transfer it to a blender. With the machine on, gradually blend in the butter until smooth, about 45 seconds. Transfer to an airtight container and refrigerate the lemon curd overnight.
Make the verbena oil and powder In a small saucepan of boiling water, blanch 1/3 cup of the lemon verbena for 10 seconds. Transfer to a small bowl of ice water to cool completely. Drain and squeeze the lemon verbena dry, then transfer to a blender. Add the grapeseed oil and puree at high speed for 4 minutes. Cover and refrigerate overnight. Strain the oil through a cheesecloth-lined fine sieve before using.
Preheat the oven to 225°. Spread the remaining 2/3 cup of lemon verbena in an even layer on a small rimmed baking sheet. Bake for about 10 minutes, until the leaves are dry and crisp. Transfer to a spice grinder and let cool, then grind the leaves to a powder.
Make the snap pea salad In a small bowl, combine the shelled peas with the lime juice and sunflower oil; season with sea salt. Remove 8 of the onion cups from the pickling liquid and blot dry with paper towels; reserve the remaining pickled onions for another use. Spoon 1/2 teaspoon of trout roe into each onion cup and top with the pea salad and lemon curd, so that the filling is flush with the top of the onion cup. Carefully flip the onion cups over. Using a small fine sieve, dust the top of the onions with the verbena powder. Use a spatula to transfer the onion cups one by one to clean plates. Place a spoonful of verbena oil on the plate. Garnish with watercress and serve.