Prepare udon noodles according to package instructions. Rinse under cold water, drain well, and set aside.
Heat a drizzle of oil in a pan over medium heat. Add sliced onions and cook until translucent.
Add beef slices and sauté until just browned. Season with chili flakes.
Add water, soy sauce, and mirin. Stir to combine.
Add curry cubes and stir until fully dissolved. Simmer until the sauce thickens slightly.
Drizzle in the eggs and let them cook gently in the curry until just set (you can lightly stir for egg ribbons or leave them as soft pockets).
Add udon back in.
Place udon noodles in a bowl and pour the curry over the top.
Top with green onions and any extra vegetables you like.