Blistered tomatoes and a crunchy panko topping elevate this three-cheese mac.
Preheat broiler. Slice tomatoes into thick rounds, drizzle with oil, and season with salt and pepper. Broil until tomatoes start to brown, about 8 minutes; let cool slightly. Store, covered, in the refrigerator up to 1 day.
Preheat oven to 350 degrees. Boil pasta in a medium pot of boiling salted water until al dente, about 2 minutes less than per package instructions. Drain, reserving 1/4 cup pasta water.
Return empty pot to medium heat and add butter. When butter melts and foams, add flour and whisk until darkened slightly, 1 to 2 minutes. Add mustard powder and whisk to combine. Slowly whisk in milk. Bring to a boil, then reduce heat to medium and cook, stirring often, until thick enough to coat the back of a spoon, 3 to 4 minutes.
Reduce heat to low and add cheese in 3 batches, starting with American. Follow with 2 cups cheddar and 1 cup Parmesan. Whisk to fully combine after adding each cheese. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Toss pasta with cheese sauce and reserved pasta water. Transfer to an 8-inch baking dish.
In a bowl, mix panko with melted butter and remaining 1/4 cup Parmesan and 1/4 cup cheddar. Top pasta with broiled tomatoes and panko mixture. Bake until bubbly, about 30 minutes.