Combine the burger ingredients together. Chill for 5 minutes in the freezer.
Mince the parsley, cilantro, mint and chop the nuts. Mix into chilled burger mix.
Form meat into golf ball size balls. Using your finger, make a hole into the patty, add a small half teaspoon of feta, and form the meat around the feta. Gently form into 2-inch patties. Using your thumb, make a small dimple in the middle of the patty. Place on a slightly oil baking sheet.
Cooking the burger
Preheat oven to 375°F
Heat a fry pan over medium-high heat. Working in batches, sear burgers 1 minute on each sides, drain on paper towels then transfer to a sheet pan. Transfer the burgers to the oven to finish cooking, about 4 minutes for medium rare.
Preparing the tahini
Mix together all ingredients in a blender, and pulse till it forms a smooth paste
Split and toast buns. Assemble burgers with tomatoes, onions, lettuce and a smear of tahini sauce.