Mix together garlic, miso, soy sauce, chili powder, salt and pepper. Put the water into a pan, add the seasonings and onion. Bring to a boil and cook over low heat for 10 minutes. Add the crab, chiles, zucchini and scallions, cover the pan and cook over moderate heat for 10 minutes. Uncover and remove pan from the heat. Let stand for 10 minutes move covered. Serve warm with rice.