Mix together soy sauce, sake, mirin and dashi. (Sake and Mirin wines can be bought in most liquor stores. Dashi, a base for Japanese soup stock, is available in most Oriental stores.) Bring to boil.Add steak and return to boil.Cook 3 to 5 minutes.Be careful not to overcook the meat;it should be rare on the inside. Remove steak, wrap in foil and refrigerate overnight. Continue boiling, cooking until liquid has reduced to a thick sauce. You can tell when it is done, because the sauce makes large, slow bubbles when it is ready.Be careful; otherwise the sauce will burn. Refrigerate overnight. To serve: Cut all fat from steaks. Slice in very thin slices. Arrange on a plate. Just before serving; pour 1 tsp. Of reduced cooking sauce over meat. You can also garnish with a little chopped green onion. For an appetizer serving, allow 5 to 7 slices per person. Serves 6.