To make the burger sauce, whisk together mayonnaise, garlic, relish, mustard, ketchup, vinegar, smoked paprika, and onion powder in a small bowl. Cover and refrigerate until ready to use.
Place lentils in a medium bowl and mash with a fork or potato masher until semi smooth. Add beef, remaining smoked paprika, garlic powder and onion powder. Season with salt and pepper, to taste. Use your hands to mix well, then divide the mixture into 12 balls and place on a plate.
Preheat a large skillet over medium-high heat. When the skillet is hot, coat with a thin layer of oil. Working in batches, place a meatball in the skillet, top with a tortilla, and press down with the back of a spatula, until the beef-lentil mixture is about a ½-inch thick. If using a 12-inch skillet, you should be able to fit four tacos at a time but adjust according to your skillet size. Cook for 3-4 minutes, until the meat is crispy. Flip the taco, top with cheese, then lower the heat, cover, and continue to cook, 2-3 minutes more, until beef is no longer pink. Remove to a plate and cover to keep warm, then repeat with remaining ingredients.
To serve, top with lettuce, tomatoes and homemade burger sauce. Enjoy!