Preheat oven to 400 degrees. Spray a large baking sheet with non-stick cooking spray; set aside.
In a small bowl, mix together the mayonnaise, lemon juice and mustard; set aside.
Heat 1 tablespoon olive oil and the butter in a medium skillet over medium-low heat. Add in the onion, salt and pepper and cook until softened and lightly browned, approximately 15 - 20 minutes. Remove from heat, stir in the whiskey, mix to combine and until absorbed. Transfer to a small, heat-proof bowl; set aside.
In the same skillet, over medium heat, add in the remaining 2 tablespoons olive oil. Add in the roast beef and garlic, and cook until heated through and the beef just begins to brown, approximately 2 - 3 minutes; set aside.
Place the flatbreads onto prepared baking sheet; evenly spread the mayonnaise mixture onto the flatbreads. Evenly divide the roast beef between the flatbreads, only covering 1/2 of each flatbread; top roast beef evenly with the reserved onions. Top entire flatbread evenly with the cheese. Place into preheated oven and bake until cheese has melted, approximately 3 - 4 minutes. Fold the cheese-only half of the flatbread over the roast beef half. Secure with toothpicks and enjoy!!