Add the remaining 2 teaspoons oil to the pan. Add garlic and crushed red pepper and cook, stirring, until fragrant, 15 to 30 seconds. Add the reserved broth mixture, Szechuan sauce, cabbage and mushrooms; cook, stirring, until the cabbage is just wilted, about 2 minutes. Reduce heat to a simmer, return the meatballs to the pan, cover and cook until the sauce is thickened and the meatballs are cooked through, 8 to 10 minutes. Serve sprinkled with scallions (if using).