For the meatballs: Mix together the beef, pork, bread crumbs, parsley, eggs, garlic, and salt and pepper in a large bowl. Let chill for 1 hour. Put flour into a bowl. Using wet hands, form meat mixture into 20 even-size meatballs. Roll each in flour; shake off excess; transfer to sheet pan.

Heat half the oil in a large skillet over medium-high heat. Brown half the meatballs in the skillet, 10–12 minutes. Transfer meatballs to a plate. Wipe out skillet and repeat with remaining oil and meatballs, leaving oil and caramelized bits in skillet.

For the sauce: Heat skillet (with reserved oil) over medium-low heat. Add garlic, onions, leeks, and bay leaf and cook until softened, 12–15 minutes. Add flour; cook for 2 minutes. Whisk in broth and wine, raise heat to medium-high, and bring to a boil while whisking constantly. Reduce heat to medium-low; simmer until thickened, 12–14 minutes. Let cool; discard bay leaf. Purée sauce in blender in batches. Return sauce to skillet along with meatballs; bring to a boil over medium-high heat. Reduce heat to medium-low; simmer until thickened and meatballs are cooked, 16–18 minutes. Season with salt and pepper. Divide between four 6″–7″ cazuelas.