Fill the center of each mushroom with a quarter of the arugula, ¼ cup of the pizza sauce and 1 tablespoon of goat cheese. Then place 1 tablespoon onion, 1 tablespoon of portobello stems, a quarter of the sliced cherry tomatoes and couple fresh basil leaves on top of each.
Bake for 15-20 minutes. Remove from the oven, sprinkle on a little freshly grated Parmesan cheese, top with fresh basil, drizzle with balsamic vinegar and serve.
Cheese: Feel free to use a different cheese option or use dairy-free if you need this recipe to be vegan.