Fill a deep, heavy pot with a lid about one-third full with oil, and heat it until a deep-fat thermometer reads 375°F/ 190°C.
In the meantime, combine the rice flour, salt, pepper, and garlic in a medium bowl.
Put the fish sauce in a small bowl.
Dip each piece of crab very briefly into the fish sauce, gently shaking off excess, and then into the rice flour mix.
Roll the crab over and shake off any extra flour. Set aside. Repeat this process, until all the crab halves are dredged.
When the oil reaches 375°F/ 190°C , gently lay the crabs, top side down, in the oil. Don’t crowd the pot—if necessary, fry them in batches—and use the lid as needed when the crabs are first added to the oil to avoid splattering.
After 1 to 2 minutes, when the crabs are golden brown, turn them over and cook for another 2 minutes.
Drain on clean brown paper bags and eat hot.