Heat oil in a frying pan over medium-high heat. Add onion, stirring until softened and translucent.

Add the chicken and cook until it is almost cooked through.

Add dashi and bring to boil. Skim the excess fat from the surface of the soup.

Stir in curry roux, soy sauce, and salt until the roux is fully melted.

Mix water and cornstarch to make a slurry (optional). Pour the slurry into the frying pan. Stir together until everything is well combined and thickened. Turn off the heat and set aside.

Serve cooked udon in a bowl, pour over the curry, and top up with green onion/scallion.