Precook the sweet potato until just tender, but not mushy (I microwaved mine for 4 minutes, but you could also bake it for 30 minutes or so in a 375 degree oven.) Remove the peel, and break up the sweet potato with knife or fingers, into very small pieces. 

Combine the ground turkey, sweet potato, spinach, and seasonings into a large bowl, and form into four patties.

Cook on the grill, pan, or griddle over medium high heat, for about 3-4 minutes on each side, or until the burgers are cooked through. (You can also bake these in a muffin tin in the oven at 350 degrees for 15-20 minutes, or until they reach an internal temp of 165 degrees or more.) 

To serve, place on a bun or lettuce wrap and top with your favorite toppings and condiments.