Wash romaine lettuce thoroughly under cold water. Dry using a salad spinner or clean kitchen towel. Remove core and shred or chop into bite-size pieces until you have 2 cups. Place in a bowl and keep chilled until assembly.

Slice tomato

Rinse tomato and remove stem. Using a sharp knife, cut into thin 1/8–1/4 inch slices. If making for sandwiches or burgers, trim ends and cut slices to fit bread if needed. Pat slices lightly with paper towel to remove excess moisture and set aside.

Slice onion

Peel the red onion and cut into thin half-moon slices. Separate into rings or smaller pieces as desired. If onion flavor is sharp, soak slices in cold water for 5 minutes then drain and pat dry to mellow flavor.

Prepare pickles

Drain pickle jar and lay out desired number of pickle slices (6 slices indicated). Pat briefly to remove excess brine if you want less wetness on the sandwich.

Warm and season mushrooms

Heat a small nonstick skillet over medium heat. Add 1 tsp olive oil. When oil shimmers, add 1/2 cup thinly sliced button mushrooms in a single layer. Cook, stirring occasionally, for 2–3 minutes until softened and edges begin to brown. Season with 1/8 tsp salt and a pinch (1/16 tsp) freshly ground black pepper. Remove from heat and let cool slightly — mushrooms can be served warm or at room temperature.

Prepare sweet peppers

If using raw sweet peppers, core and seed then slice into thin strips and measure 1/4 cup. For a softer, slightly roasted flavor, toss strips with a light brush of olive oil and roast under a broiler or in a 425°F (220°C) oven for 6–8 minutes until edges char lightly. Let cool. If using jarred roasted sweet peppers, simply drain and slice to 1/4 cup.

Prepare cherry peppers

Drain jarred cherry peppers and slice into rounds or thin strips until you have 2 tbsp. Keep juices if you want extra tang for assembly, or pat dry for less wetness.

Prepare banana peppers

Drain and slice pickled banana peppers into rings until you have 2 tbsp. Pat dry if needed.

Assemble and season

Arrange toppings in separate small bowls or on a sheet for self-serve assembly. Combine or season leftovers as desired: toss shredded lettuce with a light pinch of salt and pepper if using immediately. Keep tomatoes and onions chilled until ready to top sandwiches. Warm mushrooms should be added last to avoid wilting other fresh toppings.