01Prepare sauce: In an ovenproof pot or Dutch oven over medium-high heat, warm oil. Add onions and sauté until soft, about 6 minutes. Add garlic, and cook for 1 minute longer. Stir in tomatoes and salt and reduce heat to medium-low. Simmer sauce, occasionally mashing tomatoes against side of pot with a wooden spoon, until they break down and sauce thickens, about 25 minutes. Let cool slightly.
02Pass sauce through coarsest disc of a food mill, or purée it until smooth in a food processor or blender. Return sauce to pot.
03Prepare meatballs: Soak baguette in enough water to cover until soft and falling apart. Squeeze dry. In a very large skillet over medium heat, warm 3 tablespoons oil. Add onions and sauté until translucent but not browned, about 8 minutes. Add garlic, stir well, and turn off heat. Let cool.
04Heat oven to 325 degrees. In a large bowl, combine lamb, pork, cheese, parsley, mint, pepper, salt, red pepper flakes, onion mixture and bread. Using your hands, mix well. Form mixture into 1 3/4-inch meatballs.
05Add remaining oil to pan, and warm it over medium-high heat. Fry meatballs, turning on all sides, until well browned. Transfer meatballs to platters lined with paper towels.
06Add wine to skillet, and let it simmer for a few minutes, scraping up any browned bits stuck to bottom of pan with a wooden spoon. Pour wine into tomato sauce and add meatballs. Cover pot and bake for 30 minutes.
07Just before serving, heat broiler. Brush baguette slices with olive oil. Toast them under broiler until golden around edges, about 1 to 2 minutes per side. Serve meatballs with sauce, crostini and extra cheese.