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. 

Remove it from the freezer and let it sit on the counter for 5 minutes. Hold the döner log with a piece of baking paper and carefully make large thin slices 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.