Optional toppings: refrigerated tzatziki sauce, fresh baby spinach and tomato slices
8 whole wheat hamburger buns, split
2 pounds lean ground beef (90% lean)
1/4 tsp. pepper
1/2 tsp. salt
3/4 tsp. Greek seasoning
2/3 cup crumbled feta cheese
3 garlic cloves, minced
2-1/2 cups fresh baby spinach, coarsely chopped
2 shallots, chopped
1 tsp. olive oil