Preheat oven to 350°F. Spray a 13x9-inch baking dish with nonstick cooking spray.
Steam broccoli florets until fork tender. Drain well.
In a medium-size skillet sauté onion and celery in 3 Tbsp butter over medium-high heat. Cook just until tender and beginning to brown, around 2-3 minutes. Remove from the heat.
To a large bowl add steamed broccoli, cooked onion and celery, chopped chicken, cream of chicken soup, half and half or heavy cream, sour cream, mayo, Dijon, granulated garlic, seasoned salt, black pepper and onion powder. Mix to combine.
To the bowl add 1 1/2 cups shredded cheddar cheese. Stir to evenly distribute. Spread evenly in baking dish. Sprinkle the top of the chicken mixture with the reserved 1 cup cheddar cheese.
Toss together Ritz crackers and 1 Tbsp melted butter in a medium bowl. Sprinkle over the cheddar cheese.
Transfer to oven and bake for 35-45 minutes or until bubbly and golden brown.
Let stand for 5 minutes then serve with mashed potatoes or rice.
Store leftovers chilled in an airtight container in the fridge for up to 3 days.