Combine all ingredients for the marinade in a mixing bowl- lemongrass, shallot, garlic, fish sauce, soy sauce, red pepper flakes, salt, sugar, and oil. Whisk together to combine.
To prepare the chicken- with breast side down, locate the backbone. Using a very sharp pair of kitchen shears, cut down each side of the spine to remove the backbone. Then flip the chicken back to breast-side up and press on the sternum until chicken flattens out.
Place chicken in a non-reactive dish and pour marinade over the top.
Rub the marinade into the chicken and be sure to get some garlic and lemongrass under the skin for extra flavor.
Marinate for at least 1 hour and up to 24 hours in advance.
Next, preheat your oven to 350 degrees.
Pre- heat a grill pan to high heat. Sear the chicken breast-side down for 4-5 minutes or until blackened sear marks develop, then flip.
Insert a meat thermometer into the thickest part of the thigh.
Place the entire pan in the oven and roast, uncovered, for about 35-40 minutes or until the thermometer reads 165 degrees.
Serve garnished with cilantro.