Combine ginger, soy sauce, mirin, sake, and sugar in a small bowl. Heat a nonstick frying pan over medium high heat and cook the ground beef, breaking it up into small pieces. We don't want any big chunks here! Actually, the pictures you see above are too big of chunks. I actually like mine a little more chunky, but that's technically not the correct way. Do what you want! You can either use chopsticks (a bundle of chopsticks is the traditional method), or a whisk to break the meat up into small pieces. Once the meat is nearly cooked through, add the sauce and stir until the beef has absorbed the sauce and the pan is mostly dry. Serve over hot rice and top with green onion and egg. I poached mine, but you can sunny-side-up it, of throw just a raw egg yolk on there!