1-Step 1: Preheat and Prepare Your Baking Dish Preheat your oven to 350 degrees Fahrenheit. This moderate temperature allows the potatoes to cook through completely without burning the top. While the oven heats, generously butter a large casserole dish. I use a 9×13-inch dish for this recipe, which holds all those layers perfectly. The extra butter on the dish prevents sticking and adds flavor to the potatoes that touch the edges those crispy corners are often the best part!

2-Step 2: Sauté the Aromatics and Ham Heat 2 tablespoons of butter in a large skillet over medium heat. Add the chopped yellow onion and cook for 1 to 2 minutes until translucent and fragrant. You want them soft but not browned they should smell sweet and mild. Next, add the 3 cups of diced ham to the skillet. Cook the ham for 3 to 4 minutes until it’s heated through and has released some of its savory oils into the pan. This step wakes up the ham’s flavor and ensures every bite tastes seasoned rather than plain. Remove the skillet from the heat and set it aside.

3-Step 3: Prepare the Cream Sauce Here’s where the magic happens. Warm 1 1/2 cups of half-and-half and 1 1/2 cups of heavy cream in the microwave for about a minute just until warm, not boiling. Warm cream blends more smoothly with the flour. Whisk in the 1/4 cup of flour and black pepper to taste, whisking constantly until no lumps remain. The flour needs to be fully incorporated so your sauce is silky smooth. Add salt sparingly at this point ham is naturally salty, and the cheese will add more seasoning as it melts.

4-Step 4: Combine the Cheeses In a medium bowl, mix together the 1 cup of grated cheddar cheese and 1 cup of grated Monterey Jack cheese. Blending these two cheeses gives you the best of both worlds: the sharp bite of cheddar and the meltability of Monterey Jack. If you have time, grate your own cheese rather than using pre-shredded freshly grated cheese melts more smoothly and tastes better because it doesn’t contain anti-caking agents.

5-Step 5: Slice Your Potatoes Slice your 3 pounds of potatoes very thinly about 1/8 inch thick. A mandoline makes this job quick and ensures uniform thickness, which means even cooking. If you don’t have a mandoline, a sharp chef’s knife works too; just take your time to keep the slices consistent. Uniform slices are crucial because thin pieces cook at the same rate, preventing some potatoes from being mushy while others stay crunchy.

6-Step 6: Layer the Casserole Now comes the fun part building your layers! Divide your ingredients mentally into thirds for three complete layers. Start by arranging one-third of the potato slices in the bottom of your buttered casserole dish. Top with one-third of the ham and onion mixture, spreading it evenly. Sprinkle one-third of the cheese blend over the ham, then pour one-third of the cream mixture evenly across the top. Repeat this layering process two more times, ending with cheese and the remaining cream mixture on the very top. The top layer of cheese will create that gorgeous golden crust everyone fights over. Make sure the cream mixture seeps down into all the nooks and crannies gently press the potatoes down if needed to submerge them slightly.

7-Step 7: Bake Covered, Then Uncovered Cover the casserole dish tightly with aluminum foil and bake for 40 minutes at 350°F. The foil traps steam, which helps the potatoes cook through. After 40 minutes, carefully remove the foil the steam will be hot! Bake for another 20 minutes uncovered, or until the top is bubbly and lightly browned. You can check doneness by inserting a knife into the center; it should slide through the potatoes with no resistance.

8-Step 8: Rest and Serve Let the casserole rest for about 10 minutes before cutting into squares. This resting period allows the sauce to thicken slightly so your servings hold together nicely. Sprinkle with chopped fresh parsley if you want a pop of color and freshness. Serve warm and watch it disappear!