Delicious Broccoli Casserole

1 large head fresh broccoli
1 (10 1/2 oz.) can cream of mushroom soup
2 T. minced onion
1/2 c. sour cream
1 1/2 T Dijon mustard
1 T. horseradish (optional)
1/2 cup melted butter
1 cup dry stuffing mix

Preheat oven to 350. Cut off broccoli florets and smaller stems, discarding large stems. Parbroil broccoli. Drain and place in 9"x9" casserole dish. Mix all ingredients except broccoli, melted butter, and stuffing on top of stove on medium heat until melted. Pour sauce over broccoli. Lightly toss stuffing mix in melted butter. Sprinkle over top of sauce. Bake covered for 30 minutes, until bubbly. Uncover for last 15 minutes. Serves 6.