Preheat the oven to 350 degrees F and grease a 9x13" casserole dish.
Melt the butter in a large skillet over medium heat. Add the cauliflower rice and the broccoli. Season with salt and pepper, then cover and cook until the broccoli turns a bright green, stirring occasionally. (if you are using frozen vegetables, you will want to thaw them first, then just cook them in the skillet until they are heated through)
Stir in the chicken, chicken broth concentrate, heavy cream, sour cream and 2 cups of the cheddar cheese. Stir to combine, then transfer to prepared baking dish. Top with remaining 1/2 cup of cheese.
Bake for 15-20 minutes or until cheese is melted and slightly browned, and the casserole is hot and bubbly.