Parboil the spinach and rinse in cold water (or thaw frozen spinach, which has already been boiled). Drain and chop into small strips.
Cut carrots and zucchini into matchsticks. Cut onions into a medium dice.
Cook noodles in plenty of water. If you’re using the traditional Korean sweet-potato noodles or similar “glass” noodles made of other foods (bean thread, rice, etc.), watch them carefully; some only need to soak in hot water, and others require just a minute or two of active boiling before they turn to mush. You want them slightly chewy so that they’ll soak up seasonings in the sauté stage. Rinse noodles in cold water, and cut them with kitchen scissors into lengths your family can handle.
Fry carrots and onions gently together in a lightly oiled pan until soft but not brown, about 10 minutes. You can add the zucchini halfway through frying, or wait and toss it in raw.
Just before serving, sauté the noodles with sesame oil, soy sauce, sugar, and sesame seeds. When noodles have absorbed liquids, toss them with the other ingredients, season to taste, and serve. Serves 4-6.