Make the burgers: In a large bowl, combine burger ingredients. Shape into 8 patties and refrigerate until ready to cook. Once ready to cook, grill burgers over a medium-high heated grill, turning occasionally until meat is no longer pink, about 15 minutes. Keep burgers warm.
Make the feta: Place cheese, spinach and garlic in a food processor. Process a 30-60 minutes or until feta is smooth and creamy. Set aside.
Make the onions: In a small saucepan over medium-high heat, saute onion in butter 1 minute until onions are just barely softened, Stir in remaining ingredients and continue cooking, stirring occasionally until onions are softened and caramelized.
Assemble the burgers: Place burgers on split buns. Top burgers with whipped feta, caramelized onions and bun tops. Enjoy immediately!