In a bowl, combine fish sauce, soy sauce, lime juice, brown sugar, minced garlic, and red chili flakes (if using). Add the sliced beef and toss to coat. Marinate for 10-15 minutes.
Heat vegetable oil in a skillet over medium-high heat. Add the marinated beef and cook for 3-4 minutes until it’s browned and cooked through. Season with salt and pepper to taste. Set aside to cool slightly.
Fill a shallow dish with warm water. Dip one rice paper wrapper into the water for 5-10 seconds until it softens. Carefully remove it and place it on a clean surface.
In the center of the softened rice paper, place a few basil leaves, cilantro leaves, a few strips of cucumber, carrot, and bell pepper. Add a few strips of cooked beef on top.
Gently fold the sides of the rice paper in, then roll it tightly from the bottom up to enclose the filling. Repeat with the remaining rice paper wrappers and filling ingredients.
Arrange the rolls on a platter. Serve with a dipping sauce made of lime juice, fish sauce, and a touch of sugar, or any dipping sauce of your choice.
