Remove the gizzards and any internal organs inside the chicken's cavity. Under cold running water, rinse chicken well. Thoroughly dry the chicken inside and out.
In a food processor, pulse garlic until smooth. In a small bowl, combine pureed garlic and pepper.
Generously rub the chicken inside and out with rock salt. Massage the chicken inside and out with the garlic and pepper mixture. Let stand for about 20 to 30 minutes.
Remove and discard the dark green layers of the lemongrass until you get to the lighter green part. With a knife, cut into lengths that will fit inside the chicken's cavity. Using the back of the knife, pound ends of lemongrass to release flavor.
Peel the onion and cut one end to level.
Stuff lemongrass into the chicken cavity. Insert onion with cut side facing outwards.
In a large, thick-bottomed pot, add rock salt to about 1 1/2 to 2 inches deep. Line banana leaves on top of the salt.
Position chicken on an upright, "sitting" position on banana leaves.
Cover pot with a tight-fitting lid. Over medium heat, cook chicken for about 1 1/2 to 2 hours or until a thermometer inserted in the thickest part reads 165 F.
Gently lift chicken from pot and arrange on a serving platter. Serve hot with dipping sauce.