Add buckwheat to a large bowl, pour in some water, just so that it covers the buckwheat and leave overnight. If you don't have the time, then place the buckwheat into a pot and cook for about 5 minutes in salted boiling water. Drain and set aside.
Cut the tomatoes into smaller chunks. Place into a baking dish (approx. 20 x 30 cm / 8 x 12-inch). Add the tomato passata, season with salt, pepper, oregano and a tablespoon of olive oil, stir to combine. Cover with aluminum foil and place in a preheated oven. Bake for about 30 minutes at 210°C / 410°F.
Prick the eggplant several times with a fork. Then you can 1) hold it with tongs directly over the grill flames and cook it for about 10 minutes, turning regularly or 2) cook it for about 10 minutes outside on a barbecue. You want to cook the eggplant until it is totally black on the outside, burnt. Wrap in plastic wrap, set aside for about 5 - 10 minutes for the skin to get loose, then peel it with your hands.
Place the buckwheat to a large bowl. Add the balsamic vinegar, 1/4 tsp of harissa (optional, but highly recommend it), chopped parsley, olive oil, chopped garlic, tahini (optional, but highly recommend it) and shredded feta. Stir to combine, season with salt and pepper.
Fill each eggplant
Remove the tomato sauce from the oven and carefully place each (whole) eggplant on top of the sauce. Using a sharp knife, cut the eggplant lengthways, but make sure not no cut it all the way through. Fill each eggplant with a generous amount of the buckwheat mixture.
In a small bowl combine the breadcrumbs, parmesan cheese and one tablespoon of olive oil. Generously sprinkle over the stuffed eggplants. Place in the oven and bake (broil) for about 10 minutes at 210°C / 410°F.
In a small bowl combine Greek yogurt, 1/4 tsp of harissa (optionally). Serve eggplant with buckwheat, feta cheese and tomato sauce with a generous dollop of Greek yogurt and sprinkle with some fresh, chopped parsley. Enjoy.