As fun as it is to dip grilled cheese in tomato soup, it'd be a whole lot cooler to just combine it all into one dish!
Preheat your oven to 350°F. With a serrated knife, make lengthwise cuts 1 inch apart in a bread loaf, cutting to but not through the opposite side. Make crosswise cuts 1 inch apart in the loaf, again to, not through, the opposite side, creating “cubes” in the loaf. We'll come back to this.
Mix together the Cheddar, mozzarella, and basil in a medium bowl, and put it aside.
Melt the butter in a small saucepan on medium heat, then add the garlic. Cook that stuff while stirring often until it becomes fragrant. Add the soup and pepper, and continue cooking, switching to an occasional stir, until it's heated through. Carefully pour the soup mixture evenly into the cuts in the bread loaf, then stuff the cheese mixture into the same cuts.
Transfer the stuffed bread loaf to a baking sheet, and let it sit for 15 minutes. Cover the loaf loosely with foil, and bake it in the preheated oven until it turns golden and bubbly. Serve immediately.