Combine tomatoes, onions, garlic, vinegar, salt, pepper, and basil together. Refrigerate for 3-4 hours for flavors to combine. Lightly toast bread slices. With a slotted spoon divided tomato mixture between bread slices. Sprinkle each bread slice with 1 teaspoon Parmesan Cheese. Broil on high until cheese is slightly melted and edges of bread are browned. Serves 6.