Preheat the oven to 425 degrees . Arrange the garlic bread on a baking sheet and bake until toasted; let cool.
Meanwhile, scoop out the tomato flesh and seeds and coarsely chop; transfer to a small bowl. In a large bowl, toss the tomato halves with 3 tablespoons olive oil; season with salt and pepper. Arrange cut side up on a foil-lined baking sheet, sprinkle with 1 tablespoon basil and bake for 30 minutes.
Meanwhile, cut the garlic bread into cubes. Using a food processor, process into crumbs; transfer to a medium bowl.
In a large skillet, heat the remaining 2 tablespoons olive oil over medium-high heat. Add the onion and garlic and cook, stirring occasionally, until softened; season with salt and pepper. Add the tomato flesh and cook until most of the liquid is absorbed, about 3 minutes. Add the shrimp and cook, stirring often, until heated through. Add to the garlic breadcrumbs and stir in the remaining 1 tablespoon basil.
Preheat the broiler. Divide the shrimp mixture among the roasted tomato halves and top with the cheese. Broil until the cheese is melted, about 2 minutes.