In a large bowl mix together the buttermilk, egg, lemon juice, and chicken thighs. Let the chicken thighs soak in the buttermilk mixture for at least 15 minutes.
In another large bowl, combine the all-purpose flour, cumin, paprika, allspice, chili powder, salt and pepper.
Take the chicken thigh, shake off the excess buttermilk, and dredge it through the flour. Repeat process with remaining three chicken thighs.
Preheat a skillet to medium-high heat and add the olive oil. Add the chicken thighs and fry on the first side about five minutes. Flip, and continue to cook for another four minutes or until cooked through and golden brown. Remove from skillet and drain the chicken on a paper towel.
To serve, take a hamburger bun and place the chicken thigh on the bottom. Garnish top of the chicken with red onion, butter lettuce, tomato slices, and greek yogurt sauce. Top with remaining bun and serve.