Add the onion puree, diced chorizo, piquillo peppers and butter beans to the skillet and cook, stirring, until sizzling, about 2 minutes. Add the chicken stock and crumbled saffron, season with salt and bring to a boil. Stir in the rice. Nestle the chicken legs into the rice, cover and cook over moderately low heat until the rice is nearly tender, about 18 minutes. Add the peas, mussels, clams and shrimp, cover and cook until the shellfish is open and the chicken is cooked through, about 8 minutes. Let the paella rest for 5 minutes before serving.