Put the saffron in small pot with 1 1/2 cups stock and about 1 cup water. Bring to boil over medium heat, then reduce the heat to low and let steep for a few minutes while you prepare the chorizo and onions.
Heat a skillet with a tight fitting lid over medium-high heat. Remove the casings from the chorizo and slice about 1 to 1 1/2 inches thick, on an angle. Add a drizzle of extra-virgin olive oil to the hot skillet and add the chorizo. Brown evenly on both sides, 4 to 5 minutes total. Remove the chorizo to a plate and add another drizzle of oil to the skillet along with the onions. Season with salt and pepper, to taste, and cook until softened, about 5 minutes. Add the pimientos and peas and stir to heat through. Deglaze the pan with sherry or wine and stir 1 to 2 minutes. Remove them from the pan to the plate with the chorizo.
Return the pan to medium heat and melt 2 tablespoons of the butter. Add the pasta and stir until deeply golden brown, about 3 to 4 minutes. Stir in the rice and stock and bring to a boil. Reduce the heat to low, cover the pan and cook for 18 minutes. Fluff the rice with a fork.
Grease a casserole with the remaining 1 tablespoon of softened butter. Fill the casserole with cooked rice and top with an arrangement of the chorizo and onion mixture. Cool and chill as a make-ahead meal. Reheat by dousing the casserole with 1/2 cup chicken stock then putting it in preheated 375 degree F oven, loosely covered, for 20 minutes. Uncover and bake for about until heated through and barely crispy at edges, about 10 minutes. Top with parsley and serve.