Place the tomato slices in a large ziploc bag. Pour the vinegar over the tomatoes in the bag & gently toss to coat. Place the bag in the fridge & allow the tomatoes to marinate for an hour or two, tossing the tomatoes once or twice while they are marinating.
Preheat the oven to 375 degrees. Line a large baking sheet with foil for easy clean up.
While the oven is preheating, add the spinach and garlic to a skillet. Top with the olive oil & then saute over medium heat until the spinach has just wilted.
Spoon a small amount of the spinach and garlic over each tomato slice. Top with shredded mozzarella cheese & then sprinkle grated parmesan cheese on top.
Bake for 10-15 minutes or until the cheese is melted and the tomatoes have softened.