Preheat an oven to 300 degrees F (150 degrees C). Grease a 7-inch casserole dish.
Melt the butter in a skillet over medium heat. Stir in the bell pepper, onion, zucchini, squash, and chicken broth. Bring to a simmer, and cook until the vegetables have softened, about 10 minutes. Use more broth if needed to keep the mixture moist. Remove the vegetables from the heat, and stir in the condensed mushroom soup, sour cream, and 3/4 of the stuffing mix; pour into the prepared casserole dish. Sprinkle the remaining stuffing mix over top.
Bake in the preheated oven until browned on top, about 30 minutes.
