Wash and dry all produce. Peel onion, then thinly slice crosswise and separate into rings. Slice tomato into thin rounds. In a small bowl, combine mayonnaise and hot sauce (to taste).

Heat a drizzle of olive oil in a large pan over medium heat. Add onion and cook, tossing, until softened and browned, 4-5 minutes. Season with salt and pepper. Remove from pan and set aside.

Rub mushrooms all over with a large drizzle of olive oil. Season with salt and pepper. Add to same pan over medium heat. Cook until just tender, 4-5 minutes per side. Pour 11/2 TBSP balsamic vinegar into pan (we sent more) and cook until reduced by half. Toss to coat mushrooms, then remove from pan and set aside.

Wash and dry same pan, then return to stove over medium heat. Split buns in half down center. Add to pan cut side down. Toast until golden brown, 3-4 minutes.

Halve, pit, and scoop flesh from avocado, then roughly chop. Place in a small bowl, then mash with a fork until mostly smooth. Season with salt and pepper.

In a medium bowl, toss arugula, a drizzle of olive oil, remaining balsamic vinegar, and a pinch of salt and pepper. Spread spicy mayo on buns, then add a mushroom to each bun. Top with tomato, onion, and avocado. Serve with salad to the side.