Pat the chicken thighs dry with paper towels and season all over with 1 teaspoon salt and ½ teaspoon black pepper.
Heat 2 tablespoons olive oil in a large heavy-bottomed pot or Dutch oven over medium-high heat. Once shimmering, add the chicken thighs skin-side down and sear 5–7 minutes per side until golden brown. Remove and set aside.
Reduce heat to medium, add the chopped yellow onion and green bell pepper, and cook 5–7 minutes until softened, stirring occasionally.
Stir in the minced garlic, 2 tablespoons tomato paste, 1 teaspoon ground cumin, and 1 teaspoon smoked paprika. Cook 1–2 minutes until fragrant.
Add the rinsed long-grain white rice and stir to coat in the vegetable mixture. Pour in 4 cups chicken broth, the saffron with its soaking liquid (2 tablespoons warm broth), and add the bay leaf. Stir to combine.
Bring to a gentle boil, then nestle the seared chicken thighs skin-side up into the liquid. Reduce heat to low, cover tightly, and simmer 20–25 minutes until the rice is tender and the chicken reaches 165°F.
Remove from heat, stir in the frozen peas and sliced roasted red pimientos, then cover and let rest 5–10 minutes.
Fluff the rice with a fork, taste and adjust seasoning if needed, and serve hot garnished with chopped fresh cilantro (optional).