Fill a pot with about 4 liters (approximately 4 1/4 quarts) water. Peel ginger and cut into small cubes. Peel and chop garlic. Chop chiles. Peel and dice onions. Rinse lemongrass, remove tough outer layer, trim ends and cut 2 cm long (approximately 3/4 inch long) pieces.
Add ginger, garlic, chiles, onion, lemongrass, 100 ml (approximately 7 tablespoons ) soy sauce and 50 ml (approximately 1/4 cup) fish sauce to the water and bring to a boil. Cook at a full boil for about 5 minutes, then reduce the heat. Add crabs to the boiling water and simmer 15-20 minutes. Lift crabs out of the pot with a slotted spoon. Crack shells gently with a small hammer or a lobster mallet and remove meat. Serve crab meat drizzled with cooking liquid.