Preheat the oven to 350 degrees F (175 degrees C). Heat 1/4 cup olive oil in a skillet over medium heat. Add onions and cook until soft, about 5 minutes. Add garlic and cook 1 to 2 minutes more, being careful to avoid burning. Remove from heat and allow to cool. Whisk basil, eggs, and parsley together in a bowl. Add salt and pepper. Combine ground turkey, Italian sausage, bread crumbs, Parmesan cheese, Italian cheese blend, 2 tablespoons oil, balsamic vinegar, and Worcestershire sauce in a large bowl. Add cooked onion mixture and egg mixture and mix using your hands until completely mixed together. Shape meat mixture into a large loaf in a 9x13-inch baking dish. Pour marinara sauce on top. Bake in the preheated oven until an instant-read thermometer inserted into the center reads at least 160 degrees F (70 degrees C), about 1 hour.