In a bowl, combine the sliced beef with chopped lemongrass, garlic, oyster sauce, and sesame oil. Mix well to ensure every piece of beef is coated in this fragrant marinade. Allow it to marinate for at least 15 minutes to absorb the flavors.
As the beef marinates, prepare the rice vermicelli noodles according to the package instructions. Drain and set aside.
In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the sliced onion and bell pepper, cooking until they are tender and slightly caramelized, about 5-7 minutes. Remove the vegetables from the skillet and set aside.
In the same skillet, add another 2 tablespoons of butter and increase the heat to medium-high. Once the butter is melted and bubbling, add the marinated beef in a single layer. Cook for about 3-4 minutes until browned, flipping halfway through. Avoid overcrowding the skillet; you want to sear the beef for the best flavor.
Return the sautéed vegetables to the skillet with the beef. Stir everything together and cook for an additional 2-3 minutes until everything is heated through.
Fill a shallow dish with warm water. Briefly dip each rice paper wrapper in the water until it softens, about 10-15 seconds. Lay the wrapper flat on a plate.
Place a leaf of lettuce on the softened rice paper, followed by a generous scoop of the beef and vegetable mixture. Top with fresh cilantro and sliced jalapeños. Fold the sides of the wrapper over the filling, then roll it up tightly, like a burrito.
Arrange the Viet Fajitas on a platter with additional lettuce, herbs, and jalapeños on the side. Serve with your favorite dipping sauce for an extra kick!
