In a small bowl, whisk together 3 tablespoons mayonnaise, 1 teaspoon prepared horseradish, 1/2 teaspoon Dijon mustard, 1/2 teaspoon lemon juice, 1/8 teaspoon salt, and a pinch (1/16 tsp) of black pepper until smooth and uniform. Taste and adjust horseradish for heat. Set aside.

Prepare components

If using store-bought horseradish mayo, measure 2 tablespoons and set aside. If you made the mayo above, use 2 tablespoons from the batch.

Crisp the pastrami: heat a skillet over medium-high heat. Once hot, add the 2 oz pastrami in a single layer (no oil required). Cook 1–2 minutes per side until edges are browned and slightly crisped. Transfer to a plate and keep warm.

Prepare avocado: halve and pit the avocado. Scoop out the flesh from half (about 1/2 avocado), place in a small bowl, and mash lightly with a fork. Stir in 1/2 teaspoon lemon juice, 1/4 teaspoon salt, and 1/8 teaspoon black pepper. Keep the other half for another use.

Slice the tomato and thinly slice the red onion. If onion is sharp, soak slices in cold water for 5 minutes, then drain to mellow the bite.

Toast and assemble

Toast the challah: spread about 1/2 teaspoon butter on one side of each slice of challah. Heat a skillet over medium heat or preheat a griddle. Place bread butter-side down and toast 1–2 minutes until golden and crisp. Flip and toast the other side briefly (about 30–45 seconds) so bread is warm through but not burnt.

Spread 1 tablespoon horseradish mayo on the inside of each toasted slice of bread.

Layer the sandwich on one slice of toasted challah in this order: 2 oz smoked turkey (arrange to cover bread), crisped pastrami (2 oz), mashed avocado (spread evenly over meat), tomato slices (2 slices), red onion (about 2 tablespoons), and lettuce leaves (2 leaves).

Season the tomato and avocado lightly with a pinch of salt and a few grinds of black pepper if desired.

Top with the second slice of challah, mayo-side down. Press gently to compact.

Cut the sandwich in half diagonally (or into thirds if preferred) and serve immediately.