Lamb - Ground
In a bowl, combine oats, egg white, pepper and salt. Add lamb; mix well. Form into eight 1/4-inch patties. In a small bowl, toss together the feta cheese and green onion. Place one-fourth of the cheese mixture on each of four patties. Top with remaining four patties and seal edges to hold filling inside.
In bowl, combine vinegar, mustard and sugar; mix well. Pour over chopped spinach; toss to mix. Set aside.
Grill burger over medium-hot coals for 5 to 6 minutes per side until no longer pink inside. Place each burger on a muffin half; top with spinach mixture and second half of the muffin.