Remove the stems from the mushrooms. Discard the stems and place the mushrooms stem side down on a 15 x 10-inch baking pan with sides. Bake for 5 minutes.
Meanwhile, cook the sausage in a medium skillet over medium heat until thoroughly cooked; drain if necessary. Stir in the breadcrumbs, cheese and 2 tablespoons of the sour cream.
Turn the mushroom caps stem side up and brush with oil.
Heap the sausage mixture onto the mushrooms (about 1 tablespoon each). Bake for 7 to 10 minutes or until hot and bubbly.
Sprinkle the mushrooms with the green onion and dollop with the remaining sour cream.