Preheat the oven to 450°F.

Using a spoon, remove the stems and the gills from the mushrooms, which are the dark ridges on the underside of the mushroom.

Heat the olive oil in a skillet over medium heat. Add the shallot, red pepper and zucchini and sauté for 1 minute. Add the cleaned and trimmed spinach and cook until tender, about 2-3 minutes.

Spray the bottom of each mushroom with nonstick cooking spray and place on a baking sheet. Place in the oven for 1-2 minutes just to heat the mushrooms. Rub each mushroom with the smashed garlic. Divide the vegetable mixture between the mushrooms and top with the tomatoes and cheese.

Bake for 6 minutes or until the cheese is melted and bubbly. Drizzle vinegar over cooked mushrooms before serving.