Place all ingredients except the oil into a mixing bowl and mix together with your hands.
Form the mixture into 4 patties.
Preheat a large skillet over medium heat, and add the olive oil to the pan.
Place the burgers into the skillet and cook for 6 minutes each side, or until the center of the burgers reaches 165F degrees when tested with a thermometer. Enjoy!
Notes
Go easy on the salt. Feta is naturally salty, so don't add any extra.
Reheat in the oven, not the microwave. To reheat, place the patties in an oven-safe dish at 350°F until warmed through — this preserves the texture far better than the microwave.
Use fresh spinach, not frozen. Fresh spinach gives these burgers better texture and quality. If frozen is all you have, thaw it completely and squeeze out as much moisture as possible before adding it to the mix.