Heat the oven to 390°F, or 350°F fan. Toss the potatoes, carrots, red onion, garlic, olive oil, salt, and pepper in a large roasting pan.
Coat the chicken thighs with olive oil. Mix the paprika, garlic powder, onion powder, thyme, salt, and pepper, then sprinkle the seasoning over the chicken.
Arrange the seasoned chicken skin-side up over the vegetables.
Whisk the flour with a little beef stock until smooth. Whisk in the remaining stock, dark soy sauce if using, and gravy pepper, then pour the mixture into the pan around the chicken.
Bake for 45 minutes, until the chicken is cooked through, the skin is browned, the vegetables are tender, and the liquid has thickened into gravy.
Transfer the chicken to a plate. Stir the thawed peas into the hot gravy and vegetables, then return the chicken or serve immediately.