Set oven to 350 degrees (set oven rack to second-lowest position).
Pam spray a 2-quart casserole dish (or one large enough to hold the mixture).
In a skillet heat water over medium heat; add in onion, celery and green bell pepper; cook stirring over medium heat until softened (about 5 minutes, adding water as needed) or until well softened adding in garlic the last 2 minutes of cooking, place the mixture into the bowl.
Add in soup, milk, drained canned mushrooms mayonnaise, sour cream, black pepper, garlic powder and cooked cold rice; mix with a wooden spoon until well combined adjusted black pepper and adding in a small amount of salt to taste (I use seasoned salt).
Transfer to prepared casserole dish.
Sprinkle parmesan cheese evenly over the rice mixture.