In a large mixing bowl, combine the ground beef, fresh baby spinach, crumbled feta cheese, sun-dried tomatoes, egg, and a pinch of salt. Mix until well incorporated and shape into four patties.

Cover the patties with plastic wrap and refrigerate for at least one hour.

Preheat a skillet or grill over medium heat. Brush the cut sides of sandwich buns with olive oil and toast for 2-3 minutes.

Increase heat to medium-high, add oil if needed, and cook patties for 4-5 minutes on each side, or until internal temperature reaches 160°F.

Assemble the burgers on toasted buns with your choice of toppings and serve immediately.