In an oven safe skillet over medium heat, brown the turkey. While browning and breaking meat up with a wooden spoon, add in the salt, pepper, sage, nutmeg, red pepper flakes, and ground ginger. Cook until no longer pink.
Stir in chopped spinach and tomatoes. Cook for about 2-3 minutes or until the spinach begins to wilt. Gently stir in the egg whites, only stirring around once or twice to combine everything. Sprinkle the top with feta cheese. Cook over medium to medium-low without stirring, about 5 minutes. The top will still be liquid, but the sides and bottom should be firm.
Place skillet on the center rack of the oven, turn on the broiler to finish cooking the top. Watch carefully to avoid burning, this should take 2-3 minutes. Slice and serve.
For the turkey sausage, I just make a whole pound of ground turkey (double the seasoning), and just use half the meat in the frittata and save half for another breakfast (freezes great too). Just easier that way since ground turkey is generally sold by the pound.