In a medium-sized pot, combine 4 cups of water and 3 tablespoons of white miso paste. Whisk vigorously over medium heat until the miso is fully dissolved.

Once the broth is simmering, stir in 1 cup of heavy cream. Continue cooking over low heat for another 2–3 minutes.

Prepare the udon noodles in a separate pot according to the package instructions, typically about 8 minutes until al dente.

Return to the simmering broth and add 8 ounces of sliced beef brisket and 1 cup of sliced mushrooms. Cook for 4–5 minutes.

Once the beef and mushrooms are tender, drain the udon noodles again and add them to the broth. Stir carefully to combine.

Ladle the soup into bowls, topping each with a perfectly poached egg. Optionally garnish with sliced green onions or sesame seeds.