Season the potatoes well, but don't oversalt. Put 1 tablespoon oil in a medium to large skillet over medium-high heat. Add the onion and cook, stirring frequently until it softens, about 5 minutes. Crumble the meat and add it in, stir to break up clumps. Cook another 5 mins. Turn off the heat, then add 1 tablespoon of the soy sauce, salt and pepper. When the meat is cool enough to handle, combine with the potatoes. Taste and adjust the seasoning as needed. Form into 4 large or 8 small patties. Put the butter and the remaining 2 tablespoons oil in high heat. When the butter melts, dredge each patty in flour, then run through the egg, repeat until coated. Flip them over a few times. Combine the remaining 1/4 cup sopy sauce with lemon juice.