In a small mixing bowl, combine tomatoes, onion, garlic, basil, oregano, and salt and pepper. Mix well, cover, and refrigerate.
Preheat oven to 375 degrees F (190 degrees C).
Cut the loaf of French bread into 12 slices on the diagonal. Arrange the bread on a baking sheet and toast in the oven for 5 minutes, or until golden brown. Reduce the oven's temperature to 250 degrees F (120 degrees C).
Spoon equal amounts of the tomato mixture onto the toasted slices of bread. Top with slices of mozzarella. Place the pan of bread back into the oven until the cheese has melted just slightly and is oozing over the tomatoes, approximately 2 minutes. Serve immediately.