Step 1

1 For the pickled onion: Place the onion in a medium-size heatproof bowl

Step 2

2 Combine the vinegar, water, sugar, salt, thyme and black peppercorns in a small saucepan over high heat

Step 3

3 Bring just to a boil, then pour over the onion, making sure it is submerged

Step 4

4 Let it sit for 1 hour, then drain, reserving the liquid for another use, if desired

Step 5

5 For the herb Dijon mayonnaise: Whisk together the plant-based mayo

Step 6

6 Dijon mustard, capers, parsley, dill and lemon juice in a medium bowl

Step 7

7 Taste; season with salt as needed

Step 8

8 You'll need a total of 6 tablespoons for the sandwiches; the rest can be refrigerated in an airtight container for up to 5 days

Step 9

9 For the sandwiches: Spread each half of the bun with 1 tablespoon of the herb Dijon mayonnaise

Step 10

10 Cut three 3/4-inch slices from middle of the peeled tomato

Step 11

11 Place a slice of tomato on each bottom bun half

Step 12

12 Season the tomato lightly with salt and pepper, then drizzle with a little oil

Step 13

13 Top each portion with avocado slices, 2 tablespoons of the pickled onion and some alfalfa sprouts

Step 14

14 Top with the remaining brioche bun halves

Step 15

15 NOTE: To peel the tomato, score a shallow X in the bottom

Step 16

16 Place in a pot of boiling water for 20 to 30 seconds, then immediately drain and transfer to an ice-water bath

Step 17

17 When cool enough to handle, discard the tomato skin