Heat the oven to 375°F and grease a 9-by-13-inch baking dish.
Toss the tomatoes with 1/4 teaspoon salt in a colander and let them drain for 10 minutes.
Spread half the bread in the dish and add half the tomatoes, basil and mozzarella, then repeat the layers.
Whisk the eggs, milk, garlic powder, remaining salt and pepper until smooth and pour the custard evenly over the bread.
Press the bread gently into the custard, let it stand for 15 minutes and sprinkle the Parmesan over the top.
Bake for 40 to 45 minutes, until puffed, golden and 160°F at the center.
Rest for 10 minutes before cutting.