Preheat oven to 350 degrees. Butter six 8-ounce ramekins or a 2-quart baking dish. Microwave 1 tablespoon butter until melted, then toss with cereal and Parmesan.
In a medium saucepan, melt 2 tablespoons butter over medium. Add onion and cook until softened, 4 minutes. Sprinkle with potato starch and cook 1 minute. Slowly whisk in milk and cook, whisking constantly, until thickened, about 3 minutes. Remove from heat and stir in cheddar and mustard until smooth; season with salt and pepper. Add pasta and stir to coat; transfer to ramekins or baking dish. Sprinkle with cereal mixture and bake until sauce is bubbling and topping is golden, 15 to 20 minutes.
Pasta for All!
The wide variety of noodles in the market makes it easy to go gluten-free. See our guide to alternative pastas.