Put buckwheat in a sieve and rinse under running water. Bring to the boil in a pot with 400 ml salted water. Cover and simmer at low heat for about 12 minutes. Remove from the heat and let it swell for about 5 minutes. Loosen buckwheat with a fork.
Halve the peppers lengthwise, clean and wash. Wash zucchini, cut off ends. Cut the zucchini in quarters crosswise. Scoop out the inside with a spoon or ball cutter, leaving a small base. Unscrew the stems of the mushrooms. Wash and clean the mushrooms.
Finely dice mushroom stems and courgette insides. Peel and finely dice onion and garlic. Wash and chop the rosemary. Heat oil in a pan, fry minced meat crumbs while turning. Fry onion, rosemary, mushroom stems, zucchini interior and garlic briefly. Season with salt and pepper. Add Ajvar, deglaze with 1⁄4 l water, bring to the boil and stir in broth. Simmer for 2-3 minutes. Stir in buckwheat. Season the filling with salt and pepper.
Place the vegetables on an oiled tray and fill with buckwheat mixture. Spread any remaining filling on the tray next to the vegetables. Bake in the preheated oven (electric oven: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 15 minutes. Mix yoghurt and lemon juice. Serve with the vegetables.