Chop the onion into small pieces by hand or in a food processor, and place both onions and crushed clove of garlic into a large nonstick skillet over medium heat, with a drizzle of extra-virgin olive oil and butter and sauté until soft.
Add to the pan, rough chopped spinach and cook only until the leaves shrink and turn bright green.
Transfer the spinach mixture into large bowl, add the lemon juice, zest, teaspoon of oregano, pepper flakes, salt and ground meat. Mix well.
Gently mix in the crumbled feta and begin to form the mixture into a good burger shape, either 6 medium size burgers or 4 huge burgers.
Heat the same large skillet on medium-high heat with a little extra-virgin olive oil, and place the burger patties into the pan to cook on both sides until golden brown. The best burger has an internal temperature of 160 to 165 for a well-done burger, less if you prefer it medium rare.
Serve with a burger bun and perhaps some Tzatziki sauce.