Make the pickled onions: Place the sliced onions in a small tight-lidded jar. Add the white wine vinegar, cane sugar and salt. Fill the jar with enough water to cover the onions. Gently shake and chill for at least 20 minutes. This will keep in the fridge for several weeks if the onions are covered by the liquid.
Make the sandwich filling: Place the edamame, cilantro, lemon juice, scallions, garlic, and sea salt into the bowl of a food processor. Pulse until chopped. Add the olive oil and blend until smooth. Add up to 3 tablespoons of water to create a smooth, spreadable consistency. Season to taste and chill until ready to use. The filling can be in advance and stored in the fridge for 3 to 4 days.
Spread the edamame sandwich filling on 4 slices of bread. Top with the cucumber and avocado slices, pickled onions, arugula and the remaining bread slices.
Recipe by Love and Lemons at https://www.loveandlemons.com/lemon-edamame-avocado-sandwich/