In a medium bowl, combine the diced tomatoes, red onion, basil, balsamic vinegar, olive oil, garlic, salt, and black pepper. Mix well and set aside to let the flavors meld for about 15 minutes.
In a separate bowl, beat the softened cream cheese until smooth. Add the mozzarella, Parmesan, and Italian seasoning, mixing until well combined.
Preheat your oven to 350°F (175°C).
Spread the cream cheese mixture into the bottom of a baking dish. Spoon the tomato mixture evenly over the top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is bubbly and the top is lightly golden.
While the dip is baking, toast the baguette slices on a baking sheet for about 5-7 minutes, or until golden brown.
Serve the bruschetta dip warm with the toasted baguette slices for dipping.