Heat the pan. Pour in the oil. Wait until fine wisps of smoke float on the surface.
Throw in the chayote and squash strips. Over HIGH HEAT, stir fry for about three to four minutes.
Add the chilies. Sprinkle with salt and pepper. Stir fry for another minute.
Add the mushrooms, tomatoes, sliced onion, garlic and ginger. Sprinkle with more salt and pepper. Stir fry for a minute.
Add the sugar and kalamansi (or lime or lemon) juice. Pour in a tablespoonful of water. Toss a few times. Cover the pan for a minute or two.
Drizzle in the soy sauce. Toss and taste. Adjust the seasonings, if needed.