Finely grate the white onion. Place the grated onion in a sieve or cheesecloth and press or squeeze to drain out the excess water. This step is crucial as it helps prevent the kabab mixture from becoming too wet and falling apart during cooking.
In a large bowl, combine the ground beef and ground lamb. Add the drained-grated onion, turmeric, salt, black pepper and sumac to the meat. Mix in the saffron to evenly distribute the flavor and color.
Using your hands, thoroughly mix all the ingredients until they are well combined and the mixture is homogeneous. This helps the kababs to hold together better when cooked. Allow the kabab mixture to rest for 30 minutes.
In a small bowl, combine tomato paste and butter. Grease large non-stick skillet with the butter mixture. Transfer the meat mixture to the skillet and gently spread and flatten the mixture in the skillet, pressing it down to form an even layer about ¼ inch thick across the skillet's surface.
Dividing the meat into 1 to 1 ½ inch thick strips within the pan using a spatula. This will make the kababs easier to turn and ensure they cook evenly.
Cook for about 7-8 minutes on one side. Then, carefully turn the strips over to cook on the other side for another 7-8 minutes or until both sides are nicely browned and the meat is cooked through.
Heat a couple of tablespoons of vegetable oil in a small frying pan and pan-fry the cherry tomatoes and chilli peppers. Alternatively, preheat the oven to 180°C. Arrange the cherry tomatoes and chilli peppers on a baking tray. Drizzle with olive oil and season with salt and pepper. Roast in the oven for 10 to 12 minutes until softened and slightly charred.
Add the pan-fried cherry tomatoes and sweet chilli peppers over the kabab. Top with a sprinkle of sumac for an extra burst of flavour. Serve with fluffy Persian rice on the side.