Mix garlic, coriander roots, black pepper and 2 tbsp. oil. Grind the mixture finely in a mortar and mix well in a food processor. Now spread evenly over the steak.
Heat the remaining oil in a frying pan until very hot. Place the steak in the frying pan and roast for 4 minutes on each side, turning only once to the other side during the roasting time. Remove the steak from the frying pan and cool.
Rinse the lettuce and separate the leaves, halve the tomatoes, roughly dice the cucumber and chop the spring onions.
For the dressing: combine soy sauce, lime juice, fish sauce, chilies and brown sugar in a small baking dish form and stir until sugar dissolves.
Cut the cooled steak into narrow strips. Place the lettuce leaves on a serving platter, arrange the cucumber, tomatoes, scallions and steak strips on top.
Drizzle with the dressing and sprinkle the cilantro leaves on top. Bring to the table immediately.