Wash the eggplant, peel off the skin and cut in small cubes. Place the cubes in a stockpot add enough water to cover and add salt. Cover and let the eggplant cook for no more than 5 - 7 minutes. Make sure the pieces do not overcook. Remove the eggplant pieces from the liquid, mash, and set aside. Reserve the eggplant broth to use later in the recipe.
Heat the oil in a saucepan or saucepan over medium heat and add the garlic, tomatoes, onion, and peppers. Stirring occasionally, let the mixture cook/fry for about 2 - 3 minutes. Add the chayote and let cook for 5 minutes while stirring. Add eggplant, continue stirring and make sure all the ingredients are well mixed. Add the oregano, lemon juice, black pepper and salt to taste.
Let the vegetable mixture cook by stirring it occasionally. Make sure that eggplant and chayote do not stick to the pan. Occasionally add water (use the liquid/broth from the eggplant) if necessary. Fully cook the vegetables (eggplant and chayote before adding the watercress.
Add the chopped watercress, and stir. Continue to cook the mixture until the watercress has wilted. Remove the pot from the heat and serve hot with rice, pasta or any other types of grains.
Reserve eggplant cooking liquid to add to the stew and use instead of regular water if possible.