20 lg. mushrooms
1 (10 oz.) pkg. frozen chopped spinach
1/4 c. diced onion
1/3 c. plain low-fat yogurt
1/2 c. grated cheddar cheese
1/2 c. grated Swiss
1/2 c. grated Parmesan cheese
1/4 tsp. garlic powder
1/4 tsp. oregano

Clean mushrooms and remove stems. Remove wrapper from spinach and microwave for 5 minutes or until thawed. Drain well and press between paper towels. Combine with remaining ingredients. Spoon mixture into mushroom caps. Put a dot of butter on top of each. Microwave on high for 5 minutes until heated thoroughly or place in pan under broiler in oven until heated and browned lightly. Serve immediately.