Preheat your oven to 350°F (175°C). Peel and dice the potatoes, then boil them in salted water for about 10-12 minutes until they’re fork-tender. Drain and set them aside. While the potatoes are boiling, steam the broccoli florets for 5-7 minutes, just until they’re tender but still vibrant green. For the chicken, you can use pre-cooked, shredded rotisserie chicken for convenience, or cook chicken breasts and shred them yourself.
In a medium mixing bowl, whisk together sour cream, milk, melted butter, garlic powder, salt, and pepper until it forms a smooth and creamy consistency. This sauce will coat the ingredients and make the casserole irresistibly creamy. You can also add a pinch of smoked paprika or cayenne pepper for an extra kick if you like some heat.
In a large mixing bowl, combine the boiled potatoes, steamed broccoli, and shredded chicken. Pour the creamy sauce over this mixture and gently stir until everything is well-coated. Add in half of the shredded cheddar cheese and mix it through. This will ensure that every bite of the casserole is cheesy and creamy.
Lightly grease a 9×13-inch baking dish with olive oil. Pour the assembled casserole mixture into the dish, spreading it evenly. Top with the remaining shredded cheddar cheese and sprinkle the grated Parmesan cheese on top for a rich, golden crust. If you want a crunchy topping, you can add breadcrumbs here as well.
Bake in the preheated oven for 25-30 minutes. The cheese should be fully melted, and the casserole should be bubbling around the edges. If you added breadcrumbs, you may want to turn the broiler on for an additional 2-3 minutes to get a crispy, golden top. Keep an eye on it to prevent burning.
Let the casserole sit for 5-10 minutes before serving. This helps it set and makes it easier to serve. You can pair it with a side salad or your favorite vegetable dish for a complete meal.