Heat oven to 350 degrees F.
Spray both sides of bread and press bread into small 12 cup muffin tin. If you do not own a muffin tin, disposable tins are available on the baking aisle of market. Place bread in oven and toast until golden, 7 or 8 minutes. Remove and reserve.
In a skillet, melt butter into oil. Add onions and bay leaf, season with thyme, salt and pepper. Cook onions until caramel colored, 15 to 18 minutes.
Turn broiler on.
Place spoonfuls of cooked onions in toasted bread cups. Discard bay leaf. Cover onions with cheese and set tarts under hot broiler to bubble and brown cheese. Serve hot.