Finely dice the cilantro roots as fine as possible, or use a mortar and pestle to grind.
In a sealable plastic bag add the cilantro roots, soy sauce, peanut oil, sesame oil, black pepper, and sugar. Mush together to create a thick liquid/thin paste.
Prepare the steak by removing any excess fat, gristle, or silverskin. Place in the bag with the marinade and make sure it is smeared to cover the whole steak. Place in a cool place to marinate for 45 minutes.
To Cook
Preheat a skillet or wok, then add a drizzle of peanut oil. Add the steak and sear on every side until well browned. At this point, the outside is cooked but the inside is still raw.
Add to the skillet or wok the water and ginger to braise the steak until medium-rare, about 10 minutes. Remove from the heat and rest, reserve the braising liquid.
To Serve
Thinly slice the cucumber and decorate the serving plates with it.
Thinly slice the steak against the grain, and serve evenly onto the two serving plates.
Top the steak and cucumber with the braising liquid as a sauce. Garnish with green onions and cilantro leaves.