In a bowl, combine soy sauce, mirin, brown sugar, sake, sesame oil, grated ginger, minced garlic, and black pepper. Add thinly sliced beef and marinate for 20–30 minutes (can skip marination if you want a fast dinner).

Heat a pan over medium-high heat. Sauté onions until soft, then add marinated beef with its marinade. Cook until browned and caramelized. Set aside.

Cook udon according to package instructions. Drain and separate.

In the same pan, melt butter and sauté garlic. Stir in soy sauce, oyster sauce, sugar, mirin, and dashi or pasta water. Let it simmer briefly, then toss in udon noodles. Mix until glossy and coated. Finish with sesame oil.

Plate the noodles, top with beef and onions, and garnish with optional toppings like egg yolk, furikake, green onions, and chili oil.