Drain yogurt using a clean dish towel or a few layers of cheesecloth. Squeeze as much liquid out of the yogurt as possible. Place yogurt in the fridge overnight to drain further (the easiest way to do this is by setting a mesh strainer on top of a bowl, then placing the yogurt-filled towel/cheesecloth inside the strainer).
Once the yogurt has drained, remove from the cloth and place in bowl along with the white balsamic. Stir to combine. Place back in the fridge until ready to serve.
Mix the Greek seasoning, sesame seeds, fennel seeds and sea salt together in a small bowl or jar.
Preheat a grill pan over medium-high heat. Brush the pitas with olive oil and season both sides with the seasoning, leaving about 1/2 tsp for garnish later on. Grill the pitas for 2-3 minutes, flipping halfway through, until slightly crisp and lightly charred. Remove from heat and cut into wedges.
Divide yogurt into bowls, drizzle with olive oil and sprinkle with the remaining seasoning. Serve with pita chips, tomatoes, olives, cucumber slices and herbs, if using.