Season the chicken thighs generously with salt, pepper, and half of the smoked paprika. Let them rest a few minutes while you prep the vegetables.
Heat a large, deep skillet or wide sauté pan over medium-high heat. Add olive oil and sear the chicken skin-side down until deeply golden and crispy, about 5-6 minutes. Flip and brown the other side for 2-3 minutes. Remove to a plate and set aside.
In the same pan, reduce heat to medium. Add the chopped onion and a pinch of salt, cooking until translucent and just starting to caramelize, about 5 minutes. Add garlic, remaining paprika, and cumin; sauté for 30 seconds until fragrant.
Stir in the rice and toast for 1-2 minutes, ensuring each grain gets a light coating of oil and spices.
Pour in the chicken stock and crushed tomatoes. If using saffron, bloom it in a small amount of stock before adding. Bring to a gentle boil.
Nestle the seared chicken back into the pan, skin-side up. Scatter the bell peppers over the top. Reduce heat to a gentle simmer, cover, and cook for 15 minutes.
Uncover, stir gently to distribute the rice evenly, and cook for another 5-7 minutes, or until the liquid is absorbed and the rice is tender. If the liquid runs dry before the rice is done, add a splash more stock as needed.
Finish with lemon juice and chopped parsley or cilantro. Taste and adjust salt and pepper as needed. Let rest for 5 minutes off heat before serving.