Thinly slice the onion. Halve the green bell pepper, remove seeds, and slice thinly. In a bowl, mix natto with its included sauce.

Heat sesame oil in a frying pan. Add the onion and bell pepper, and stir-fry until softened.

Add the udon noodles and mentsuyu to the pan. Stir-fry to combine, then add the natto and stir briefly. Season with salt and pepper to taste.