Trim any fat from steak. Sliced thinly. Chop onion finely and combine with meat, half teaspoon pepper, half tablespoon fish sauce and cornstarch. Rinse spinach and cut each leaf into 3 pieces. Slice tomato into thin wedges and crush garlic. Heat 1.5 tablespoons of oil in a frying pan. Add remaining garlic and stir fry until fragrant. Add spinach and stir fry for 3 minutes. Add remaining fish sauce and tomato. Stir fry 2 minutes, then remove to a heated serving platter. Heat remaining oil in frying pan. Add meat and stir fry for 5 minutes. Add 2 tablespoons waterif meat sticks to pan. Remove meat from pan and sprinkle over spinach. Sprinkle with pepper and serve.