Make the filling:
In a large bowl, combine ground beef, caramelized onions, pickles, cheddar, mustard, fish sauce, salt, and black pepper. Mix until well combined. Tip: Pan-fry a small piece to taste-test the seasoning.

Prepare flatbread pockets:
Slice flatbreads in half and gently open each half into a pocket. Use a knife to help, and don’t worry about small tears—just avoid large holes.

Make the dipping sauce:
Mix yogurt, mustard, ketchup, garlic, chopped pickles, pickle juice, and smoked paprika in a bowl. Stir until well combined and set aside.

Stuff the flatbreads:
Fill each pocket with about 2 heaping tablespoons of the beef mixture, pressing it evenly so the filling is about ½ inch thick.

Cook the flatbreads:
Heat a large pan over medium-high and drizzle with olive oil. Add stuffed flatbreads and cook for 4 minutes per side, then stand them upright to crisp the open edge. Repeat with remaining flatbreads.

Serve:
Transfer to a platter and serve warm with the burger dipping sauce.