Prepare a large baking sheet by covering it in tinfoil. This will make cleanup a breeze. Lightly spray it with non stick cooking spray, and set aside.
Add the bread crumbs and broth to a small bowl, stir, and let them sit for 10-15 minutes. Just until they've soaked up the liquid.
Add the olive oil to a large skillet, and heat over medium high heat. Add the onions, tossing them to coat, and cook until the onions are soft and have become translucent.
To a large mixing bowl add the meats, onions (once cool enough to touch), prepared bread crumbs, eggs, garlic, salt & pepper, red pepper flakes, Italian seasoning, and cheese (if using).
Just like with a meatloaf, use your hands and mix everything up until evenly mixed throughout. This may take anywhere from 2-5 minutes, but make sure everything's evenly incorporated.
Continue using your hands to form the meat mixture into meatballs, each approximately 1 1/2". Place them evenly out onto the prepared baking sheet until all the meat mixture's been used.
Bake the meatballs at 425 degrees for 15-20 minutes, or until all the meatballs have cooked through and there's no pink left in the center.