Tips and Swaps: Slice beef against the grain for tenderness. Don't overcrowd the pan — sear in batches for a good crust. For gluten-free, use tamari and rice noodles. For vegetarian, substitute extra-firm tofu or mushrooms. The lo mein keeps in the fridge for up to 3 days; reheat in a hot skillet with a splash of water or soy sauce to loosen the sauce.