Mix potato starch and water in a bowl. Place the bowl in the refrigerator for 30 minutes or longer and you will see the starch sinking to the bottom and the water on top. Drain the most of water off the bowl. Now you have the paste-like potato starch.
Meanwhile slice the beef very thinly against grain and put in a large mixing bowl. Season the beef with the soy sauce, salt, black pepper, ginger, and toss well. Sprinkle sweet rice flour over the beef and toss to coat them. Set aside.
Beat the egg white until the soft peak forms. Add the potato starch paste and the beaten egg white to the beef; mix all together.
Heat oil in a wok or a skillet over medium heat, drop the beef slices and deep fry until golden. You want to do this with small batches at a time. Transfer the beef to a plate lined with paper towel. When all the beef slices are fried, repeat the deep frying again for the second time. Deep frying twice will create crispier texture. Drain off the oil on the paper towel.
For the sweet and sour sauce, combine water, vinegar, lemon juice, sugar, soy sauce, and salt in a small mixing bowl; mix well and set aside. Mix potato starch and water in a small cup and set aside.
In a skillet, heat a couple of teaspoonful of oil over medium-high heat. Stir-fry the onion, pepper, and cucumber for 60 seconds. Pour the sauce over the vegetables and let it boil, about 30 seconds. Add the potato starch water to the skillet and let the sauce thicken, about 30-60 seconds.
Place the beef on the serving plate. Either pour the sauce over the meat or serve the sauce on the side to dip. Serve immediately.