In a large bowl, mix together ground turkey, flaxseed, egg white, cranberries, and feta.
Thaw spinach and squeeze out as much liquid as possible. Add to turkey, along with salt, pepper, and cinnamon. Mix well and then shape into 4 large patties (or 6 small ones).
Place patties onto hot grill, cooking each side for 4-5 minutes until center is no longer pink.
Serve on buns with your favorite toppings. Serve watermelon on the side.