Separate the white and green parts of the green onion. Slice the white part diagonally and finely chop the green part.

Cut the pork belly into pieces 3cm wide.

Heat salad oil in a pot over medium heat. Add the pork belly and stir-fry until the color changes. Add water and bring to a boil, skimming off any foam. Add the white part of the green onion and noodle sauce, cover, and simmer on low heat for 5 minutes.

Place the frozen udon on a microwave-safe plate, cover loosely with plastic wrap, and heat according to package instructions. Transfer to a serving bowl.

Add the curry roux to the pot and stir until thickened to create the curry sauce.