Combine the Ground Sirloin, red onion, parsley, Greek seasoning, lemon juice, garlic powder, salt, and pepper in a large bowl, mixing to combine well. Add the feta, mixing lightly to incorporate. Shape into six ¾-inch-thick patties.
2. Place patties on grid over medium, ash-covered coals. Grill, covered, 8 to 10 minutes (over medium heat on a preheated gas grill, 7 to 9 minutes) until an instant-read thermometer inserted horizontally into center registers 160°F, turning occasionally. About 2 minutes before burgers are done, place buns, cut sides down, on the grid. Grill until lightly toasted.
3. Spread Tzatziki sauce on top and bottom halves of rolls. Serve burgers topped with spinach, tomato, red onions, and cucumber.
Grain & Greens Bowl Variation
Prepare and cook the burgers as directed, omitting buns. Toss 3 to 4 cups of cooked brown rice or quinoa (or blend) with 2 teaspoons extra-virgin olive oil and ¼ teaspoon each of salt and pepper. Gently fold in 5 to 6 cups of baby spinach. Divide grain-spinach mixture evenly among 6 bowls or plates. Top with burger and dollop each with 1 to 2 tablespoons of Tzatziki sauce. Arrange other veggies around the burger.
- To prepare on stovetop, heat a large nonstick skillet over medium heat until hot. Place patties in skillet; cook 12 to 15 minutes until an instant-read thermometer inserted horizontally into center registers 160°F, turning occasionally.
- To prepare in the oven, preheat to 375°F. Place burgers on a broiler pan lightly coated with cooking spray. Bake for 26 to 28 minutes, turning after 15 minutes, until an instant-read thermometer inserted horizontally into the center registers 160°F.
- Cooking times are for fresh or thoroughly thawed ground beef. Ground beef should be cooked to an internal temperature of 160°F. Color is not a reliable indicator of ground beef doneness.