Step 1

Bring a large pot of water to a boil. Add vermicelli noodles and cook until softened, 12 minutes. Drain noodles and rinse with cold water, stirring to separate the noodles. Set aside.

Step 2

Combine lemongrass, soy-based seasoning, sherry, brown sugar, and garlic in a bowl. Marinate flank steak in mixture, tossing evenly, and let sit for 30 minutes.

Step 3

Meanwhile, make sweetened fish sauce. Pour warm water into a small bowl; add sugar and lemon juice. Stir until sugar is dissolved. Stir in fish sauce, Thai peppers, and garlic. Adjust to taste. Set aside.

Step 4

Heat a large skillet over medium-high heat. Cook sliced flank steak until firm but slightly pink in the center, 5 to 6 minutes per side. Arrange cooked vermicelli noodles in bowls for serving. Place steak on top and garnish with Thai basil leaves, cilantro, and bean sprouts. Pour sweetened fish sauce over the top.