Preheat oven to 400 degrees. Warm stock, saffron, and ½ teaspoon salt in a saucepan over medium heat. Season chicken with 2 teaspoons salt, ½ teaspoon pepper, and the pimenton. Heat oil in a 14-inch paella pan or a high-sided ovenproof skillet over high heat. Brown chicken for 6 to 8 minutes; transfer to a plate.
Pour out all but 1 teaspoon oil. Cook chorizo over medium-high heat until lightly browned, about 2 minutes. Add onion and garlic. Cook until soft, about 5 minutes. Add tomatoes. Cook, stirring, until thick, about 5 minutes. Stir in rice, then add broth mixture. Bring to a boil. Cook for 2 minutes, stirring often. Add peas, then chicken. Bake for 20 minutes. Let stand for 10 minutes.
