Preheat gas grill.
In a large mixing bowl combine beef, pork, onions, salt and pepper; mix it all together.
Shape the hamburger meat into 1/4-pound patties, about 1/2 inch thick, and about the diameter of a hamburger bun.
On half of the patties make a wide indentation in the center.
In a small bowl combine feta cheese and garlic powder.
Fill the indentations with the feta mixture.
Cover and seal with the rest of the patties.
Place your burgers on hot grill.
Cook for 7 minutes; turn and cook for another 5 to 7 minutes, or until done.
In the meantime, prepare the salad.
In a large bowl combine all ingredients and stir gently until thoroughly combined.
Take burgers off the grill and place on buns.
Top with prepared salad.