Boil water in a pot, cook somen noodles according to package instructions, rinse under cold water, and drain.

Remove the stems and seeds from the red and yellow bell peppers, then cut them into bite-sized pieces.

Heat salad oil in a frying pan, add the bell peppers, and stir-fry over medium heat for 2 minutes. Remove from the pan.

Mix the lemon juice, sugar, and salt in a bowl to prepare the sauce.

In a separate bowl, combine the tuna (including its oil), somen noodles, and the prepared sauce. Mix well.