I recommend spatchcocking the chicken for shorter cooking time and more even roasting. To do so, lay the chicken breast side down and use poultry shears to cut out the backbone. Flip the chicken over and use your palm to press down firmly over the breast so that the chicken lays flat. Tuck the wings back behind the breasts. Place the chicken breast side up on a baking sheet lined with aluminum foil.
Remove the outer dry layer from lemongrass stalks. Cut the stalks in half to separate the white and green parts. Lightly bruise the green parts and reserve them. Thinly slice the white parts then pound until fine with a mortar and pestle. Also pound all the garlic, shallot and ginger. Alternatively, you can blitz the lemongrass white parts, garlic, shallot and ginger in a food chopper until fine.
In a bowl, add salt, black pepper, soy sauce, fish sauce and all the pounded aromatic. Mix to combine, then spread this mixture all over the outside as well as inside of the chicken to season it. Cover and let it marinate in the refrigerator for 4-6 hours. If you don't have time, leave it at room temperature for 60-90 minutes before cooking.
Preheat your oven to 450°F (230°C). Spread the bruised lemongrass green parts underneath the chicken. Shake off the aromatics from the chicken skin. You can also tuck the aromatics underneath the chicken. Brush the chicken skin with oil, and then roast uncovered in the oven for 30 minutes or until golden and cooked through. Remove from the oven and let it rest for 5-10 minutes before serving.