In a large skillet, brown the ground beef and Italian sausage over medium heat, breaking it up as it cooks. Season generously with salt and pepper. Once browned, drain most of the fat, leaving just a bit for flavor. Add the chopped onion and minced garlic to the skillet, cooking until the onion becomes translucent and fragrant. Pour in the marinara sauce, reserving ½ cup for later, and stir to combine. Set this aside.