In a mortar, blend sugar, half of the minced ginger, garlic, half of the lemongrass, and one Thai chili into a fine paste. Combine this mixture with your thinly sliced beef, along with fish sauce, oyster sauce, and oil. Mix well and cover the bowl with plastic wrap, placing it in the refrigerator to marinate for at least one hour, although overnight is preferable for deeper flavor.

While the beef marinates, create a dipping sauce by crushing the remaining sugar, ginger, garlic, and chilies into a paste using your mortar and pestle. Mix this paste with fish sauce and a splash of water until well combined. Place the sauce in a small bowl and refrigerate it to allow the flavors to meld beautifully.

Prepare the vermicelli noodles by bringing a pot of water to a boil. Add the vermicelli and cook according to the package instructions, usually about 3-4 minutes until al dente. Drain the noodles thoroughly and rinse under cold water.

Slice the cucumber and lettuce for fresh toppings. Combine thinly sliced vegetables with a pinch of salt and sugar in a bowl. Let them sit for about 15 minutes to draw out excess moisture, then rinse well.

In a nonstick skillet, heat oil over medium-high heat. Add the remaining minced lemongrass and fry for about 2-3 minutes until fragrant. Then, add the marinated beef in batches and cook for about 5-7 minutes until charred and cooked through.

To assemble your dish, place a generous serving of the cooked vermicelli in a bowl. Layer the sliced cucumber, lettuce, and cooked beef on top. Drizzle with the prepared fish sauce, and finish with a sprinkle of fried shallots, lime wedges, and sliced chilies.