Mix the soy sauce, lemon slices and pepper together in a bowl.
Slice the steak quite thinly then submerge in the marinade for 1 to 2 hours, squidging occasionally.
Heat the oil In a wok or deep frying pan. Peel and thickly slice the onion, stir fry for a couple of minutes to soften then remove to a plate, leaving the oil.
Reheat the oil, lift the steak from the marinade and stir-fry, turning frequently to brown all over. Remove to a plate.
Stir fry the garlic to soften then pour in the marinade. Bring to the boil, return the beef and simmer for about 10 minutes before stirring the onion back in.
Season to taste, then serve immediately with rice or mashed sweet potato.