Cook the udon noodles according to package instructions, usually boiling in salted water for 5–7 minutes. Drain and rinse under cold water.

In a medium pot, combine white miso paste, cream, and enough water to create a smooth broth. Heat gently while stirring for 3–5 minutes until miso dissolves.

Add sliced mushrooms and beef brisket to the broth. Simmer for 8–10 minutes until the beef is tender and the mushrooms are cooked.

Poach the egg in a small pot of gently simmering water with a splash of vinegar for about 3 minutes.

Divide cooked udon noodles between bowls, ladle the broth over noodles, top with poached egg and garnishes like scallions.