Prepare the Chicken: Using a food processor or box grater, grate the onion. Place a fine mesh strainer over a bowl and then pour the onion puree into it. Press the puree with the back of a spoon and reserve the liquid that strains out.
In a medium bowl, add chicken thighs, onion liquid, yogurt, tomato paste, olive oil, garlic, paprika, black pepper, cumin, and salt. Using a spoon or your hands, combine all ingredients well and mix until the chicken is fully coated.
Cover a work surface with two large pieces of plastic wrap. Remove the chicken thighs from the fridge. Place half of the chicken thighs flat onto the plastic wrap, making a rectangle that is about 10-inches wide. Layer the second half of the thighs on top of the first. Starting at the bottom edge, begin rolling the chicken thighs together up to the top of the plastic wrap, creating a large sausage shape. Twist each end of the plastic wrap closed like a candy wrapper, adding an additional piece of plastic wrap if necessary. Then, tie each side with a piece of twine so it is firmly closed. Place in the freezer for at least 4 hours or overnight.
