Preheat oven to 425 F. Bring a large pot of lightly salted water to a boil. Add gluten-free pasta. Cook according to package directions, stirring occasionally, until cooked through but slightly firm. Drain and rinse with cool water to stop cooking, set aside.
In a large bowl, whisk together milk, cream, cornstarch, salt and pepper until smooth. Set aside.
Heat butter in a large saucepan over medium heat until melted. Pour in milk and cornstarch mixture, stirring to combine. Continue cooking, stirring constantly, until sauce thickens, 5-8 minutes. Slowly stir in goat cheese and 2 cups shredded cheddar. Continue stirring until melted and smooth. Toss together with cooked pasta and roasted peppers.
Pour into a casserole or 8x8 baking dish. Sprinkle with remaining cheese and bake 10-12 minutes, until cheese is melted. Broil top, watching carefully, to brown if desired.
Elbow pasta is what is traditionally used for mac and cheese but you can use any short gluten-free pasta shape.
Make sure to season the pasta water as it'll make a big difference to the flavor.
Keep the sauce on a medium heat and stir as it thickens. This can take from 5-8 minutes and don't be tempted to turn up the heat.
Leftovers can be eaten hot or cold the next day, this mac and cheese won't freeze well.