Place an inverted heavy baking tray or pizza stone inside the oven and preheat to 240°C / 465°F for at least 15 minutes.
Finely chop the onion, tomatoes, peppers and parsley. Lightly drain any excess liquid from very juicy tomatoes.
Combine ground meat, chopped vegetables, garlic, tomato paste, red pepper paste, parsley and seasonings in a bowl.
Mix until the topping becomes evenly combined and spreadable. Add 1 tablespoon cold water only if it feels too dry.
Place one lavash on a flat surface and spread one-sixth of the topping thinly and evenly across it, reaching nearly to the edges.
Transfer to the heated baking surface and bake for 6 to 8 minutes, until the meat is fully cooked and the edges are crisp.
Repeat with the remaining lavash and topping, keeping cooked pieces lightly covered.
Serve immediately with lemon, parsley and sumac onions.