Grilled Cheese And Tomato Soup Bake

Preheat oven to 400°. Melt 6 tablespoons of butter and brush onto a 9"-x-13" baking dish and both sides of bread slices. Arrange slices on two rimmed baking sheets. Bake, flipping halfway through and rotating trays, until bread is golden and toasted, about 15 minutes.  Reduce oven to 350°. Top half of toasts with 2 cups each cheddar and mozzarella cheese, then sandwich with remaining toasts. Arrange sandwiches, overlapping, in the prepared baking dish. Meanwhile, in a medium saucepan over medium-high heat, heat remaining 2 tablespoons of butter. Add garlic and cook, stirring, until fragrant, about 30 seconds. Add tomato paste and cook, stirring, until toasted, about 2 minutes. Whisk in milk and bring to a boil. Remove from heat and add in basil. Season with salt and pepper. In a large bowl, whisk eggs. Slowly whisk in tomato soup until incorporated. Pour evenly over sandwiches, then sprinkle with remaining 1/2 cup each cheddar and mozzarella. Bake until cheese is melted and golden in places, about 30 minutes. Top with basil and serve warm.