Preheat oven to 350F. Spray a glass baking dish/casserole dish with non-stick cooking spray and arrange the fish evenly in it. Mix water, milk powder, cornstarch, salt, pepper and garlic together. Place the mixture in a pot and bring to a near boil; stir constantly once it gets hot. Once you see it starting to thicken, remove from heat and add in the peas and onion. Pour the hot mixture over the fish and top with the mashed sweet potatoes. Bake until the liquid starts to bubble and the potatoes are nicely browned. Yield: 4 servings.