Preheat the oven to 375 F and add the melted butter to a 3-quart baking dish.
Dice the chicken breasts into bite-size pieces and spread over the melted butter.
Sprinkle half of the poultry seasoning and half of the thyme over the chicken.
Top with the thawed peas and carrots, onions, and pimientos.
Combine the baking mix with the onion powder and the remaining sage and thyme. Stir in the milk just until blended. Pour the baking mix mixture over the casserole.
Whisk the chicken broth with the cream of chicken soup and pour over the biscuit mix layer. Sprinkle the cheese over the casserole.
Bake the casserole for 40 to 55 minutes or until the top is golden brown and the edges are bubbly.