Parmesan-Crusted Au Gratin Potatoes and Onion

Place potatoes into a large pot and cover with water; add salt and bring to a boil. Reduce heat to medium-low and simmer until just beginning to soften, 15 to 20 minutes; drain.

Melt butter in a saucepan over medium heat; add flour and whisk until the mixture thickens, 1 to 2 minutes. Pour milk and heavy cream into the saucepan; cook and stir until smooth, 2 to 3 minutes. Melt Cheddar and Asiago cheese into the butter mixture; cook while stirring continually until the mixture is smooth and creamy, 1 to 2 minutes. Season the sauce with salt and pepper.

Preheat oven to 400 degrees F (200 degrees C). Butter a 9x13-inch casserole dish.