Cut the kohlrabi into chunks and simmer to make a broth (cover and simmer for 20 minutes, then season with the vegetarian bún bò Huế soup cube).

While the broth is simmering, prepare the other ingredients. Chop the mushrooms, green onions, Vietnamese coriander, and shallot. Blend or finely mince the garlic, shallot, and lemongrass together.

Cut the tofu into small pieces, fry until golden, then add to the broth.

In a skillet, heat oil and sauté the minced garlic, shallot, and lemongrass until fragrant. Add the mushrooms and a few pieces of kohlrabi, stir-fry, then season with soy sauce and salt. Return everything to the pot of broth and adjust seasoning to taste.
If you like, you can add a little annatto oil for color while sautéing, but this is optional and just for appearance.

Place the noodles in bowls, prepare the fresh herbs and lettuce for serving, ladle the hot broth and toppings over the noodles, add chopped green onions, Vietnamese coriander, black pepper, chili, and enjoy hot.