1. Heat oven to 375 degrees F.
2. Line a 13x9x2-inch baking pan with foil; lightly coat foil with vegetable cooking spray. Heat oil in a 12-inch skillet over high heat; add onions and mushrooms and cook, stirring occasionally, 6 to 7 minutes, until mushroom liquid has evaporated and mushrooms are browned. Cool.
3. Toss onion mixture with remaining ingredients lightly with fingertips in a large bowl. Shape mixture into a 9x4-inch loaf in pan. Cover with foil; bake 30 minutes. Uncover meatloaf and bake 25 to 30 minutes more, until an instant-read meat thermometer inserted in center registers 160 degree F. Let meatloaf stand in pan 5 minutes. Remove from pan with a large spatula and transfer to serving platter. Makes 4 servings.