In a large mixing bowl, combine the ground turkey, chopped spinach, crumbled feta, breadcrumbs, chopped onion, minced garlic, dried oregano, salt, and black pepper.
Use your hands to mix the ingredients thoroughly until well combined. Be careful not to overmix.
Divide the mixture into four equal portions and shape them into patties about ¾ inch thick.
Preheat a grill or grill pan over medium-high heat, then drizzle with olive oil to prevent sticking.
Once the grill is hot, place the turkey patties on the grill and cook for about 5-6 minutes on each side, or until the internal temperature reaches 165°F (74°C) and the burgers are golden brown.
During the last minute of cooking, place the whole wheat burger buns on the grill to toast lightly.
Assemble your burgers by placing the turkey patty on the bottom half of each bun, then top with sliced tomatoes, lettuce, and optional tzatziki sauce.
Place the top half of the bun on each burger and serve hot.