Thinly slice the shiitake mushrooms and onion.
Boil the somyeon noodles in boiling water and drain.
Heat olive oil in a pan and sauté the garlic.
Add the onion and shiitake mushrooms and stir-fry.
Add the tomato sauce and bring to a simmer.
Add the cooked somyeon noodles to the pan and mix well.
Season with salt and pepper to taste.
Transfer to a serving plate.