Gluten-Free Mac and Cheese

INGREDIENTS

• 3 tablespoons unsalted butter, plus more for dish
• 1/2 cup gluten-free crisp puffed-rice cereal, slightly crushed
• 1 ounce Parmesan, grated (1/4 cup)
• 1 small yellow onion, diced small
• 2 tablespoons potato starch
• 2 cups whole milk, warmed
• 3/4 pound cheddar, shredded (3 cups)
• 1 teaspoon Dijon mustard
• 3/4 pound gluten-free penne, cooked until al dente and drained
• unsalted butter
• Coarse salt and ground pepper