In a food processor, add mayo, feta and pepperoncini, and blend for about 2 minutes. Season to taste with salt. Set aside.
In a large bowl, mix together parsley, cilantro, mint, red pepper, garlic and red onion. Add sherry vinegar and olive oil.
In a separate bowl, combine cumin, paprika, garlic powder, onion powder, salt and pepper. Set aside.
Heat a large sauté pan on medium high heat with olive oil. Liberally rub each patty with spice rub on both sides. Cook for 4 minutes per side or until desired doneness.
Spread butter onto buns and toast in pan. Remove buns and set buns aside.
Place a large pinch of lettuce on the bottom of each bun, followed by a dollop of feta spread. Set the burger on top and finish with mint chimichurri.