1. In a bowl, combine ground beef with caramelized onions, chopped pickles, cheese, mustard, garlic, pickle juice, fish sauce, salt, and pepper. Mix well until evenly incorporated.
2. Spread about a heaping tablespoon of the mixture thinly and evenly onto each mini tortilla, pressing the meat toward the edges to create a large, thin patty.
3. Prepare the burger sauce by mixing all sauce ingredients together until smooth.
4. Heat olive oil in a pan over medium-high heat. Place the tacos meat-side down on the pan and press firmly with a spatula so the meat adheres well to the tortilla and cooks into a thin patty.
5. Cook for about 4 minutes until crispy and browned, then flip and cook for another 30 seconds on the other side.
6. Remove from heat and immediately drizzle with the prepared burger sauce. Top with chopped Romaine lettuce, additional pickles, grated cheese, extra sauce, and optional sesame seeds for added texture. Serve hot and fold like a traditional taco.