1-First Step: Get ready Preheat your oven to 350°F. Butter a large casserole dish so the potatoes won’t stick and the edges get a little brown. Wash your potatoes thoroughly; russets or Yukon Golds both work russets give a fluffier texture, Yukon Golds hold their shape a bit better.

2-Second Step: Cook the aromatics and ham Heat the 2 tablespoons of butter in a skillet over medium heat. Add the chopped yellow onion and cook for 1 to 2 minutes, until just translucent and fragrant. Toss in 3 cups diced ham and cook 3 to 4 minutes until it’s heated through and picks up some color. Set this mixture aside while you make the sauce.

3-Third Step: Make the cream sauce Warm 1 1/2 cups half-and-half and 1 1/2 cups heavy cream together about 1 minute in the microwave or a minute on the stovetop until warm but not boiling. Whisk in 1/4 cup flour and black pepper to taste until smooth. Salt is optional and should be minimal because the ham and cheeses already add saltiness. The sauce should coat the back of a spoon; if it seems too thick, add a splash more half-and-half.

4-Fourth Step: Prepare the potatoes Slice 3 pounds of potatoes very thin, about 1/8 inch, using a mandoline or slicer for even results. If you want to speed up baking and ensure tenderness, boil the slices for 3 to 5 minutes, drain, and cool slightly before layering. This parboil step reduces bake time and helps avoid crunchy centers.

5-Layering: Build the casserole Combine 1 cup grated cheddar cheese and 1 cup grated Monterey Jack cheese in a bowl and set aside. Now assemble in layers in the buttered casserole dish:* Layer one-third of the potato slices in an even layer.* Add one-third of the ham and onion mixture over the potatoes.* Sprinkle one-third of the cheese mixture over the ham.* Pour one-third of the cream mixture evenly over everything.Repeat these layers two more times, ending with cheese and cream on top. Cover the dish tightly with foil to trap steam and promote even cooking.

6-Final Step: Bake and serve Bake covered at 350°F for 40 minutes. Remove the foil and bake an additional 20 minutes until the top is bubbly and slightly golden. Let the casserole rest for about 10 minutes before cutting into squares; this helps the slices hold together. Garnish with chopped parsley if desired and serve hot.