To make yogurt sauce, combine yogurt, lemon juice, 1 clove garlic, and dill in a small bowl; mix well. Refrigerate, covered, until needed. Preheat grill or broiler to high.Combine turkey, chopped onion, sun-dried tomatoes, spinach, cheese, remaining 1 clove garlic, oregano, bread crumbs, and egg in a medium bowl. Season with salt and pepper if desired; mix well with clean hands. Form turkey mixture evenly into six patties.Grill or broil patties for about 5 minutes on each side, or until no longer pink in the middle.Serve patties topped evenly with yogurt sauce, onion slices, beet, and cucumber.