Preheat oven to 350ºF.
Place the sausage in a large skillet and brown over medium high heat, crumbling up the sausage as it cooks.
Once the sausage is cooked, remove the sausage from the skillet and place it on a plate or in a bowl lined with paper towels to absorb the excess grease.
Reduce the heat to medium.
In the remaining sausage drippings, sauté the onion, bell pepper, and celery until it begins to soften.
Add the mushrooms and garlic and sauté for another 30 seconds.
Add the soup, sausage, rice, parsley, salt and pepper to the pan and mix thoroughly.
Place the mixture in 13x9 casserole dish and sprinkle the butter pieces across the top.
Bake for about 25 minutes until the casserole is bubbly.