Combine beef, onion, garlic, oregano, paprika, cumin, egg, and almond flour. Mix and season. Refrigerate 15 minutes.
Mix feta, mint, and lemon zest together in a small bowl.
Divide beef into 8 equal portions. Press one portion flat, place a heaping Tbsp of feta filling in the center, and top with another portion of beef. Pinch edges to seal completely and form an oval patty.
Heat grill to medium-high. Grill bifteki 4-5 minutes per side until cooked through and caramelized.
Rest 3 minutes before serving.