Stir together all ingredients for the tzatziki. Cover and chill until ready to use.
Prepare the burger patties by combining all ingredients in a large bowl. Mix by hand then divide into four equal sections, roll each into a ball then press into a 5-inch round patty.
Preheat your grill to medium. Place patties on the grill a few inches apart to ensure even cooking. Cook for 5-6 minutes, then use a metal spatula to flip and cook another 5-6 minutes on the other side until desired doneness is achieved. Allow to rest on a plate (covered) for 5 minutes before assembling burgers.
Assemble the burger patties on the ciabatta buns, top with tzatziki sauce, tomatoes, onions, fresh spinach and arugula, and cucumber ribbons. Serve immediately.
