Thinly slice the beef tenderloin or flank steak.

Crush the garlic cloves.

Thinly slice the green onions.

Finely chop the red Thai chili pepper.

Grate the fresh galangal.

Combine the sliced beef, crushed garlic, sliced green onions, fish sauce, sugar, chopped chili pepper, and grated galangal in a large bowl.

Cover the bowl and refrigerate for at least 3 hours, or preferably overnight.

Trim the stems of the water spinach.

Crush the stems with a meat mallet or rolling pin.

Chop the leaves coarsely.

Bring 6 cups of water to a boil in a large saucepan.

Add the chopped water spinach and the beef mixture to the boiling water.

Simmer uncovered for 2 minutes, or until the beef is cooked through and the spinach is wilted.

Remove the soup from the heat.

Stir in the lemon juice and torn fresh mint leaves.

Season to taste with salt, pepper or additional fish sauce.

Serve hot.