It's a peasant dish, but it's pretty good. It's the Japanese version of steak + potato soup. Beef isn't really a "Japanese" thing; it came with cows when Westerners were introduced to the country. Most folks up until that time had a largely vegetarian diet, sometimes supplemented with fish or bird meats. Normally, it's just cooked with a few ingredients, beef and potatoes being the main ones. You can add onions or daikon for flavour.
NIKU-JAGA: Beef + Potato Soup
Serves 4 - Prep Time: 25mins - Cooking Time: 35-40mins
1tbsp sesame oil
1 onion, halved + thinly sliced into half-moons
1tbsp vegetable oil
10oz sirloin beef, thinly sliced and cut into 2" long pieces (almost shaved)
4 small potatoes, peeled, quartered, and soaked in water
5tbsp shoyu (soy sauce)
1" piece ginger, peeled and finely shredded
Heat the sesame oil in a deep saucepan and stir-fry the onion over high heat for about 3mins, until soft. Remove the onion from the pan and set aside. Add the vegetable oil to the pan and stir-fry the beef over medium heat, about 3-4mins until it turns pale. Drain the potatoes, add them to the pan, and stir-fry a minute longer. Return the onion to the pan.
Pour the dashi, sake, sugar, mirin, and shoyu into the pan. Stir, cover with a lid, and bring to a boil. Remove any scum from the surface (as some starches and fats may rise to the top). Lower the heat slightly and simmer for about 20 more minutes, covered, turning the bottom contents of the pot to the top once, until most of the liquid is absorbed.
Divide the cooked meat and veggies between the four bowl and shred the ginger over top. Add salt if desired.