Cheesy Au Gratin Potatoes and Onions

Start by submerging potatoes in a large container and covering them with water; add salt to taste and bring the mixture to a rolling boil. Once boiling, lower the heat to a moderate level and let it simmer until the potatoes begin to show signs of tenderization, approximately 15-20 minutes. Remove from heat and drain the water.

Next, melt butter in a saucepan over medium heat; add flour to the mixture and whisk until it thickens, taking around 1-2 minutes. Then, gradually pour in milk and heavy cream into the saucepan; cook while stirring constantly until the mixture becomes smooth, taking about 2-3 minutes. After that, melt Cheddar and Asiago cheese into the butter mixture; continue stirring until the mixture is smooth and creamy, approximately 1-2 minutes. Season the sauce with salt and pepper to taste.

Preheat your oven to 400 degrees F (200 degrees C). Take a 9x13-inch casserole dish and butter it generously.

Now, layer about half of the sliced potatoes into the bottom of the baking dish. Top the potatoes with thinly sliced sweet onions and chopped green onions; pour about 1/3 of the cheese sauce over the onions. Arrange the remaining potato slices into the dish; pour the remaining cheese sauce over the potatoes. Sprinkle Parmesan cheese and crushed crackers over the top.

Finally, bake the dish in the preheated oven until it's hot in the center and beginning to brown at the edge, approximately 30 minutes. Sprinkle some fresh parsley over the top of the dish to serve.