Melt butter with olive oil in medium saucepan over medium heat. Add onion and garlic; sauté until onion is very tender and golden brown.
Stir in tomato sauce, oregano and thyme. Simmer until thickens, stirring occasionally, about 20 minutes. Stir in Parmesan.
Smash meatballs slightly with hands to about 1/2-inch thick. Cook in large skillet over medium heat until internal temperature reaches at least 165ºF. and no longer pink.
Arrange baguette slices on baking sheet. Place under broiler to toast. Flip slices over and repeat.
Place 1 piece of cheese on 12 baguette slices. Top with large spoonful of marinara sauce, basil leaf if desired, meatball slider and another piece of cheese. Place under broiler to melt cheese. Place second slice on top and serve.