In a skillet over medium heat, brown sausage, onions, bell pepper, and mushroom stems. Add salt, garlic powder, and cayenne pepper. Add water and bring to a boil. Add rice and parsley. Cover and remove from heat. Let stand for 15 minutes.

Meanwhile, blend mayonnaise and cheese in a bowl. Add half of the mayonnaise and cheese mixture to the cooked sausage and rice mixture.

Stuff the mushrooms with this sausage mixture and spoon the remaining mayonnaise-cheese mixture on top of the mushrooms.

Arrange the stuffed mushrooms in a 9-by-13-by-2-inch baking dish and bake at 350 F for 35 minutes. Mushrooms will appear puffy and golden when done.