Place the saffron threads in a small bowl with 2 tablespoons of the warm stock and set aside to infuse while you prepare the other ingredients.
Pat the chicken thighs dry with kitchen paper and season all over with the salt and pepper.
Heat the oil in a large, heavy-based pan or casserole over a medium–high heat. Once shimmering, add the chicken thighs, skin-side down, and cook for 5–7 minutes each side until golden. Transfer to a plate.
Reduce the heat to medium and add the onion and green pepper. Cook for 5–7 minutes, stirring occasionally, until softened.
Stir in the garlic, tomato purée, cumin and smoked paprika, cooking for 1–2 minutes until fragrant.
Tip in the rice and stir to coat in the mixture. Pour in the remaining chicken stock, then add the saffron with its soaking liquid and the bay leaf. Stir to combine.
Bring to a gentle simmer, then nestle the chicken thighs, skin-side up, into the rice. Reduce the heat to low, cover with a lid and simmer for 20–25 minutes, until the rice is tender and the chicken is cooked through (internal temperature 74 °C) and the juices run clear.
Remove from the heat, stir in the peas and roasted red peppers, then cover again and leave to stand for 5–10 minutes.
Fluff the rice with a fork, taste and adjust the seasoning if needed, then serve hot, scattered with chopped fresh coriander.