Preheat the oven to 375°F/190°C.
Generously season the chicken cavity with garlic, salt, and pepper, then stuff with 2 Tbsp butter, half the onion, celery, parsley, thyme, and a few lemon slices. Truss the legs with kitchen twine if desired.
Lift some of the skin and add a few tbsp of butter. Drizzle half of the oil all over the chicken, and season it with salt, garlic, and pepper (season liberally).
Add the remaining vegetable oil to the dutch oven so it covers the bottom. Add the potatoes, remaining onions and celery, carrots, lemon (sliced), and broth to the dutch oven. Season with salt, pepper, and garlic, and give them a quick stir. Add parsley and thyme to this and mix.
Transfer the chicken to the dutch oven so it sits over the chopped vegetables. Add remaining butter and cover with the lid.
Transfer the dutch oven to the oven and bake for 50 minutes.
Remove the dutch oven from the oven, remove the lid, and cook for 30 more minutes UNCOVERED or until the internal temp reaches 165°F/74°C (use kitchen thermometer). For crispy skin, place under the broiler for 3-5 minutes if desired. (NOTE: Cook until to temp. Cook time depends on chicken weight so will vary).
Allow the chicken to rest for 10 minutes before carving.
