Wash the eggplants, cut the ends and make a big long cut for the filling.
Grease the eggplants with 1 tablespoon of oil and grill them in a big baking mold under the grill of your oven. Turn it occasionally until they change the color everywhere.
In the meanwhile (but keep an eye to the eggplants!) fry the onions in the remaining oil, when they are soft add the garlic and fry 3 more minutes.
Add the half of the tomatoes, parsley, pepper and adjust the salt to your taste.
Fill the eggplant in the baking mold with the onions mix, but let at least 3 tablespoons in the frying pan.
Adjust on every eggplant one piece of feta.
Arrange the potatoes (if used) around the eggplants.
Add the remaining tomatoes to the onion in the pan. Add some salt (about 1/2 teaspoon) and cover your eggplants and potatoes with this tomatoes sauce.
Bake about one hour at 170-180°C or until the potatoes are ready (heat and timing are basing it on convection oven). From time to time check if it is not drying to much, if necessary add 1/2 cup of water or white wine.