Remove giblets and any extra fat from inside the chicken. Under cold running water, rinse chicken well and pat dry.
Season with salt and pepper to taste. Set aside.
In a large bowl, combine calamansi juice, fish sauce, soy sauce, sugar, and 1/4 teaspoon pepper. Stir until sugar is dissolved.
Add minced garlic and chopped shallots and stir to distribute.
Add chicken to the marinade and massage until evenly coated.
Cover and refrigerate for about 4 hours to overnight, turning chicken a few times every 2 to 3 hours.
Remove chicken from the refrigerator and drain from marinade, reserving marinade.
With a knife, cut the thinner top portion of the lemongrass stalk and the very woody base. Peel off the tougher outer layers to get to the tender part of the stalk. With the back of a knife, pound the stalk a few times.
Cut the lemongrass into about 3-inch lengths and stuff into the chicken cavity.
In a saucepan, bring the marinade to a boil for about 3 to 5 minutes or until reduced and thickened.
Gently place the prepared chicken with the breast side down on the turbo broiler rack (use the lower metal rack).
Brush the chicken liberally with the reduced marinade and broil at 180 C/350 F for about 20 to 30 minutes, Occasionally brush with marinade for extra flavor.
Carefully turn the chicken to breast side up and liberally brush with the marinade. Broil for about another 20 to 30 minutes or until a thermometer inserted at the thickest part reads 165 F. Occasionally brush with marinade for extra flavor.
Turn the temp up to 215 C/420 F and continue to cook for another 10 to 15 minutes or until chicken is golden and nicely charred.
Carefully remove from turbo broiler and place on a serving platter or chopping board, Allow to stand for about 5 minutes before slicing.