Make the tzatziki sauce first. In a large bowl, combine Greek yogurt, grated cucumber, grated garlic, lemon juice, and chopped mint. Mix well and refrigerate until ready to serve.

Heat a large skillet over medium heat. Add the oil from the sun-dried tomatoes, diced white onion, and minced garlic. Cook for 3-4 minutes until the onion is soft and translucent.

Add the fresh spinach to the skillet and cook until wilted by about three-quarters. If the pan is too full, add spinach in batches. Transfer the vegetable mixture to a bowl and let it cool for 5 minutes.

Once the vegetable mixture has cooled slightly, add the ground chicken, ground pork, crumbled feta cheese, and salt. Mix everything together with your hands until just combined. Be careful not to overmix.

Form the mixture into 5-6 burger patties of equal size. Set them aside.

Heat a large skillet over medium heat and add a teaspoon of oil. Cook 1-2 patties at a time, depending on the size of your pan. Cook for 3-5 minutes per side, until browned and cooked through.

Assemble the burgers. Place each cooked burger patty on a bun. Top with red onion rounds, sliced cucumbers, and a generous dollop of the prepared tzatziki sauce. Place the other half of the bun on top and enjoy.