Set your oven to 350°F (180°C). Grease a 9x13-inch baking dish.
Melt butter in a large pot over medium heat. Season the chicken with salt and pepper, then cook until browned on all sides (work in batches if needed). Add the diced onion and celery, cooking for about 3 minutes. Stir in the broccoli and olive oil, cooking for another 3-5 minutes until slightly tender.
2 tablespoons butter, 1 pound chicken breasts, Salt and pepper, ½ cup red onion, 1 stick celery, 2 cups broccoli florets, 1 tablespoon olive oil
Remove the pot from heat. Stir in the cream of chicken soup, sour cream, milk, garlic powder, onion powder, mustard, and half of the shredded cheese. Mix everything well.
2 cups cream of chicken soup, ⅓ cup sour cream, ½ cup milk, ½ teaspoon garlic powder, ½ teaspoon onion powder, ½ teaspoon mustard, 1 ½ cups cheddar cheese
Transfer the mixture to the prepared baking dish. Sprinkle the remaining cheese evenly on top.
In a small bowl, mix the crushed Ritz crackers with melted butter. Sprinkle over the casserole.
½ cup crushed Ritz crackers, 2 tablespoons butter
Bake uncovered for 20 minutes, or until bubbly and golden on top. Let it rest for 5 minutes before serving.
Serve as is or over mashed potatoes for an extra comforting meal.