Preheat the oven to 400°F (204°C). Add the onions to a food processor and blend until smooth.
Transfer the ground meat to a large bowl and add the blended onions, tomato paste, and spices. Mix everything gently with your hands until just combined. Try not to overwork the meat, otherwise the texture can become too dense.
Divide the meat mixture into 5 equal portions. Place one portion between two sheets of parchment paper and use a rolling pin to flatten it into an even layer. The shape doesn’t need to be perfect, the important part is keeping the thickness even.
Remove the top parchment paper and carefully roll the meat tightly into a log. Transfer it to a sheet pan and repeat with the remaining meat.
On a separate sheet pan, arrange the green peppers and any other vegetables you like. Drizzle with olive oil and season with salt and pepper.
Bake the meat logs and vegetables until cooked through and nicely roasted. Let the meat cool slightly, then carefully unroll the logs and tear the meat into strips.
For the sauce, stir all ingredients together in a bowl until smooth and creamy. Add a little hot water if needed to loosen the consistency.
Serve the doner meat together with the charred vegetables and warm lavash bread. I love adding plenty of sumac onions and a spoonful of garlic yogurt sauce on the side.