Preheat oven to 400 degrees F and line a large baking sheet with parchment paper.
Remove the stems and slice each tomoato into 1/4" (or larger) slices. Depending on the sizes of tomatoes used, you may end up with minus or plus 12 slices.Place each one on the prepared sheet pan.
In a small bowl, combine the olive oil, salt, pepper, onion powder, and garlic powder.
Brush the olive oil mixture over the top of each tomato slice.
Sprinkle approximately 1 teaspoon of grated parmesan cheese over the top of each slice.
Top with shredded mozzarella cheese and basil (if preferred).
Bake for 8-10 minutes, or until the cheese has melted and started to bubble.If desired, turn the broil on for 1-2 minutes - long enough to brown the cheese.