Preheat the oven to 375 degrees and prepare a baking sheet with foil or use a nonstick baking sheet. Rinse, pat dry, and stem mushrooms. Use a spoon to carefully scoop mushroom gills out, moving in a circular path around mushrooms. (The gills will come out very easily, so no need to apply too much pressure.)

Place the Portobello mushrooms on the prepared baking sheet. Rub down both sides of mushrooms with a generous amount of pesto. Season the mushrooms with a pinch of salt and pepper. Roast and watch for 12-15 minutes. Mushrooms are done when they are tender, browned, and releasing some juice.

While the mushrooms are roasting, prepare the vegetables and salad. Thoroughly rinse produce and pat dry. Slice tomato into 1/4" slices. Peel onion and slice half of onion into 1/4" slices for topping the burger. Chop other half of onion into a small dice. Slice cucumber in half horizontally and then slice into small half rounds. Rinse and drain the garbanzo beans.

In a large bowl, whisk together red wine vinegar, 2 Tbsp. olive oil, and sugar. Add salt and pepper to taste.

Add the cucumber, chopped red onion, and garbanzo beans to the dressing bowl. Let marinate for 10 minutes. Salt and pepper to taste.

If desired, toast the Kaiser buns. Arrange bun on the plate and place a roasted mushroom on top. Dress burger with sliced red onion, tomato, and baby arugula. Add a scoop of salad on the side. Garnish with a crack of fresh black pepper.