Heat the oven to 200°C/390°F, or 180°C fan-forced.
Toss the potatoes, carrots, onion, garlic, vegetable oil, half the salt, and half the pepper in a roasting pan.
Coat the chicken with oil. Mix the paprika, garlic powder, onion powder, thyme, remaining salt, and remaining pepper, then season the chicken, concentrating the seasoning on the skin.
Arrange the chicken skin-side up over the vegetables, tucking loose pieces underneath.
Whisk the flour with about 1/2 cup of stock until smooth, then whisk in the remaining stock and soy sauce if using. Pour it around the chicken and vegetables, not over them.
Drizzle the chicken with a little oil and bake for 45 minutes, until the skin is browned, the potatoes are tender, and the chicken is cooked through.
Transfer the chicken to a plate. Stir the thawed peas into the vegetables and gravy until the sauce thickens and comes together.
Divide the vegetables and gravy among bowls, top with chicken, and serve.