Puree the onion in a food processor. Sieve its juice into a large mixing bowl. We just need the juice.

Put the ground beef, döner kebab seasoning, salt, yogurt and milk in the same bowl. Combine them well using your hand. Give it a log shape.

Transfer it on a baking paper and wrap it tightly. 

Let it rest in the fridge for 2 hours. Then transfer it to the freezer and let it sit there for 8 hours or overnight. You can remove it from the freezer when you are ready to cook.

Tomato Sauce: 

Melt butter in a saucepan. Add in tomato paste and tomato juice/puree or passata. Pour water over these. Add in salt and pepper and cook over medium heat until it boils. Reduce the heat to the lowest and let it simmer for 15-20 minutes.

While your sauce is simmering, you can cook the doner. Remove it from the freezer and let it sit on the counter for 5-10 minutes. Hold the döner log with a piece of baking paper and carefully make large thin slices (like shaving) using a sharp knife. 

Heat one teaspoon butter in a non stick pan over high heat. Line sliced döner pieces in a single layer and cook both sides until nicely brown. 

Cook döner in batches and never overload the pan. You can transfer the cooked pieces on a plate and finally put them all the pan just to heat a bit right before serving.

While you are cooking the doner, you can heat chopped pide bread in a pan with a splash of water. We want them soft, so you can add a little more water if needed.

Place heated pide bread chunks on a plate. 

Drizzle a little tomato sauce over them. 

Top it with cooked doner meat (as much as you want). 

Pour a generous amount of tomato sauce over meat.

Place a dollop of yogurt, roasted or raw tomatoes and green peppers on the side of the meat on the same plate.

Drizzle sizzling butter over these and serve immediately.