Pour boiling water over the shiitake mushrooms and soak for at least 1 hour. Then drain and collect the soaking water.
Mix 100 ml of it with the sauce ingredients.
Cut the mushrooms, bell pepper and onion into strips. Dice the carrots.
Pour boiling water over the udon noodles. Drain after 1 minute and set aside.
Heat some vegetable oil in a large pan and fry the diced carrots, peppers, onion, and mushrooms until the carrots and peppers are cooked through. Add the udon noodles and the spinach. Stir in the sauce and simmer until it is nicely thickened and the pasta is covered all around.