Preheat oven to 425°. Combine tomatoes (crushing them with your hands), garlic, anchovies, butter, and ½ tsp. red pepper flakes in a 13×9” baking dish; season with salt and black pepper. Roast, tossing halfway through, until garlic is very soft and mixture is jammy, 35–40 minutes. Using a potato masher or fork, mash to break up garlic and tomatoes.