Wash and dry all produce. Peel the onion, then slice end-to-end into thin rings. Separate the rings. Thinly slice the tomato into rounds. In a small bowl, combine the mayonnaise and as much hot sauce as you like.

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

Heat the same pan over medium heat. Rub the Portobello caps on each side with olive oil and season with salt and pepper. Add them to the pan and cook for 4-5 minutes per side, until very soft. Add 1 1/2 Tablespoons balsamic vinegar to the pan (reserving the rest for the salad) for another minute, until the vinegar reduces by half. After tossing the Portobellos in the balsamic glaze, remove from the pan and set aside.

Toast the buns: Split the buns. Clean out the same pan and heat over medium-high. Add the buns to the pan cut-side down and toast for 3-4 minutes, until golden brown.

Halve, pit, and scoop the avocado flesh into a small bowl; mash with a fork and season to taste with salt and pepper.

Finish and serve: In a medium bowl, toss the arugula with a drizzle of olive oil, the remaining balsamic vinegar, and pinch of salt and pepper. Spread the buns with the spicy mayo and top with the Portobello caps, tomato rounds, onions, and mashed avocado. Serve the arugula salad to the side and enjoy!