1. 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.

2. 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.