Heat the vegetable oil in a large pot over medium heat. Add the sliced onions and sauté until they become translucent.
Add the sliced beef to the pot and cook until it changes color and is no longer pink.
Add the potatoes, carrots, and green beans to the pot, and stir everything together.
In a separate bowl, mix together the dashi stock, soy sauce, mirin, and sugar until well combined. Pour this mixture into the pot.
Bring the mixture to a boil, then reduce the heat to low and cover the pot. Let it simmer for about 20-25 minutes or until the potatoes are tender.
Taste the broth and adjust the seasoning with salt if needed.
Once everything is cooked and seasoned to your liking, remove from heat.
Serve Nikujaga hot in bowls, garnished with chopped green onions if desired. Enjoy with a side of steamed rice.