In small bowl, stir together all of the Tzatziki Sauce ingredients, cover, chill.
In medium bowl, combine ground beef, oregano, olives and feta cheese. Shape into 2 - 4 patties, however many you want to make, place onto a waxed paper-lined baking sheet, sprinkle both sides generously with garlic powder and pepper, cover with plastic wrap, and chill.
Fire up grill and get heating. When grill is nice and hot, place burgers onto oiled grates and grill each side until browned and insides are done to your own preference.
While burgers are grilling, in perforated grill pan, add artichoke quarters or slices, and grill until lightly browned, remove to a plate. Place scrunched piles on prosciutto slices onto grill, and lightly grill each side, remove to a plate immediately. When burgers are done, top each with prosciutto piles and artichokes, and cover to keep warm.
Butter insides of buns with Garlic & Herb butter, softened or melted, and grill until toasted. Keep warm with burgers.
Heat olive oil in frying pan. Slice tomatoes, shallots and kasseri cheese. Dampen each slice of kasseri under cold water, shake off excess, and dredge in flour until lightly coated on all sides. Fry kasseri in olive oil until lightly golden on each side and starting to melt on each side as well. Remove to a plate.
Top bottom buns with baby spinach leaves, tomato, and shallot slices. Top each with a prosciutto and artichoke heart topped burger. Top each with a thick slice of fried kasseri cheese. Place onto cast iron mini fajita pans or a large cutting board, platter, or baking sheet.
One at a time, pour a 1/2 ounce of sambuco over cheese and light. Shout, "OPA!", and enjoy the flame! Let flame go out on it's own, or extinguish with a squirt of fresh lemon juice. (Flame only lasts about 10 seconds.)
Top each with tzatziki sauce, and place grilled top bun on. Take a big bite and enjoy!! "OPA!"