In a medium bowl combine ground beef, feta cheese, parsley, garlic, and pepper. Shape mixture into two 1/2-inch-thick patties.
In a large nonstick skillet cook patties over medium-high heat for 8 to 10 minutes or until an instant-read thermometer inserted into sides of patties registers 160 degrees F,* turning once.
Line cut sides of bun halves with spinach. Top with burgers, tomato slices, and Cucumber Sauce. If desired, garnish with red onion.