In a large mixing bowl, combine the diced tomatoes, red onion, basil, balsamic vinegar, olive oil, garlic, salt, and black pepper. Stir well to combine all the ingredients.
Let the mixture sit for at least 15 minutes to allow the flavors to meld.
Preheat your oven to 350°F (175°C).
Spread the tomato mixture into a baking dish and top with shredded mozzarella cheese.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
While the dip is baking, arrange the baguette slices on a baking sheet and toast them in the oven for about 5-7 minutes, or until golden brown.
Serve the warm bruschetta dip with the toasted baguette slices.