In a wok over high heat, stir-fry veggies in batches with a drizzle of sesame oil. Stir-fry zucchini, carrots and mung beans for several minutes until wilted, seasoning with salt and pepper to taste. Stir-fry shiitake mushrooms for longer; they will shrink and release a lot of moisture within a few minutes. Continue stir-frying until the excess moisture cooks off, then add soy sauce and cook for an additional minute or so, till mushrooms have soaked up sauce.