1-First Step: Prep the potatoes and chicken Start by peeling and dicing 2 pounds of potatoes into roughly 1-inch cubes for even cooking. To partially cook the potatoes, either parboil them in salted boiling water until they are tender but still firm (about 8 to 10 minutes), or place the diced potatoes in a microwave-safe covered bowl with a few tablespoons of water and microwave for 4 to 5 minutes. Drain well and set aside. If using frozen diced hashbrowns, thaw and pat dry with a towel to remove excess moisture.

2-Second Step: Make the flavored base Heat a large skillet over medium-high and melt 4 tablespoons of butter. Add 1 large finely chopped onion and 1/2 teaspoon kosher salt, then cook for about 6 minutes until soft and translucent. Stir in 2 teaspoons minced garlic and cook for 1 more minute until fragrant. This step builds the savory backbone for the sauce.

3-Third Step: Cook the roux and build the sauce Sprinkle 6 tablespoons all-purpose flour over the onion mixture and stir constantly for about 2 minutes to cook the raw flour taste without browning. Gradually whisk in 3 cups chicken broth in 1/2-cup increments, whisking after each addition to avoid lumps. Bring the sauce to a boil and let it thicken. Remove from heat.

4-Fourth Step: Add the creamy components In a separate bowl, whisk 1/2 cup sour cream and 1/4 cup mayonnaise until smooth. Slowly whisk in about 1/2 cup of the hot sauce from the skillet to temper the mixture, then pour the sour cream-mayo blend back into the skillet and stir to combine. Taste and add more salt and 1/2 teaspoon black pepper as needed.

5-Fifth Step: Combine the filling Stir in 1 cup frozen peas, 2 cups cooked chicken (shredded or cubed), and most of the parboiled potatoes into the sauce. Reserve a handful of the potatoes to arrange on top later. If your chicken is moist, drain any excess liquid first so the filling stays thick. This is a good spot to tweak seasonings or add extra vegetables.

6-Sixth Step: Assemble for baking Preheat your oven to 400°F (200°C). Grease a 9×13-inch baking dish with cooking spray or a thin layer of butter. Transfer the casserole mixture into the dish and smooth the surface. Sprinkle 1 cup grated cheddar cheese over the top, then arrange the reserved potatoes in a single layer over the cheese for a rustic look and slightly crisp top.

7-Final Step: Bake, rest, and serve Bake the assembled casserole at 400°F (200°C) until the dish is bubbly and the cheese is golden about 20 minutes. Let the casserole cool for 5 to 10 minutes before serving so the sauce thickens and the slices hold together. Garnish with minced parsley or sliced green onion if you like. A quick tip: letting it rest helps the flavors settle and makes cutting cleaner portions.