Peel daikon and cut into large bite-sized chunks. Blanch in boiling water for 15 minutes, then drain.
Cut beef into 4–5cm pieces.
Heat oil in a wide pan over medium-high heat. Sauté beef until no longer pink, about 2–3 minutes.
Add daikon to the pan. Pour in dashi water and add sugar. Bring to a boil and skim foam for 5 minutes.
Add sake, mirin, and soy sauce. Place a drop lid on the surface. Reduce to medium-low and simmer 15–20 minutes until liquid reduces by half and daikon is tender.
Serve in bowls with the braising liquid. Top with sliced green onions.