- 4 pounds russet potatoes, cut into 1/4-inch slices
- salt and ground black pepper to taste
- 6 tablespoons butter
- 6 tablespoons all-purpose flour
- 1 1/2 cups whole milk
- 1 cup heavy whipping cream
- 2 1/2 cups shredded mild Cheddar cheese
- 1 cup shredded Asiago cheese
- 1 sweet onion, sliced into rings
- 3/4 cup sliced green onions
- 1/3 cup shaved Parmesan cheese
- 7 buttery round crackers (such as Ritz®), crushed
- 2 tablespoons chopped fresh parsley
- 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 9×13-inch casserole dish.
- Layer about half the sliced potatoes into the bottom of the baking dish. Top the potatoes with sweet onion slices and green onions; pour about 1/3 of the cheese sauce over the onions. Arrange the remaining potato slices into the dish; pour remaining cheese sauce over the potatoes. Sprinkle Parmesan cheese and crushed crackers over the top.
- Bake in the preheated oven until hot in the center and beginning to brown at the edge, about 30 minutes. Sprinkle parsley over the top of the dish to serve.