For the Pickled Red Onions:
1 small red onion, halved and very thinly sliced
⅔ cup water
⅓ cup red wine vinegar
2 tablespoons granulated sugar
Pinch kosher salt
2 teaspoons fresh thyme
2 teaspoons black peppercorns
For the Herbed Mayo:
½ cup mayonnaise
1 tablespoon Dijon mustard
2 teaspoons capers, chopped
2 teaspoons finely chopped flat-leaf parsley
½ teaspoon fresh lemon juice
For the Sandwich:
1 large beefsteak tomato
4 brioche buns, split in half and toasted
2 avocados, halved and cut into 12 slices
Kosher salt and freshly ground black pepper, to taste
Olive oil, for drizzling
¼ cup pickled onions
1 cup alfalfa sprouts
1. Make the pickled red onions: Place the onions in a medium nonreactive bowl. In a small-size saucepan over medium-high heat, combine the water, red wine vinegar, sugar, salt, thyme and black peppercorns. Bring to a boil. Pour over the red onions and let macerate for 1 hour. Drain the liquid and discard the thyme and peppercorns. Makes 1½ cups.
2. Make the herbed mayo: In a small bowl, stir together the mayonnaise, mustard, capers, parsley and lemon juice. Season with salt and set aside. Makes 1 cup.
3. Make the sandwich: Prepare an ice bath. Bring a medium pot of water to a boil. Using a paring knife, cut a small "x" on the bottom of each tomato. Using a slotted spoon, lower the tomato into the boiling water and cook until the skin starts to peel back slightly, 10 to 20 seconds. Immediately transfer the tomato to the ice bath. After about 10 to 15 seconds, remove the tomato from the ice bath and use the paring knife to peel off the skin. Cut the tomato into four ¾-inch-thick slices.
4. Spread both halves of each bun with the herbed mayonnaise. Season the tomato and avocado slices with salt and pepper. Place 1 tomato slice and a few slices of avocado on each bottom bun, then drizzle with olive oil. Top each sandwich with pickled onions, alfalfa sprouts and the top bun, and serve.