Warm the pan over medium-high heat. Add the olive oil and swirl it around, then add the onions and garlic and sauté until the onions soften and glisten, about 3 to 4 minutes.
Add the sliced zucchini and cook stirring until the zucchini starts to soften, about 3 to 4 minutes. Add the bell pepper and mushrooms and continue cooking until all the vegetables are nearly fork tender, about 6 to 8 minutes.
Add the tomatoes and stir, cooking just until the tomatoes are warmed through, about 2 minutes. Stir in the basil and cook for 1 more minute, then transfer to a dish and serve with (optional) Parmesan cheese and basil leaves for garnish.