Preheat oven to 350 °.
Bring a large pot of water to boil; cook cabbage leaves, a few at a time for about 2 minutes or until softened. Drain and cool.
Heat oil over medium heat in a large skillet; sauté mushrooms, onion, bell pepper, and garlic until tender.
Add rice, salt, and pepper; stir gently until well blended.
Prepare a shallow 2-quart baking dish by brushing with vegetable oil.
Spoon mixture onto individual cabbage leaves; roll up and place seam-side down on baking dish.
Cover with foil and bake at 350 ° for 30 minutes.