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.
Recipe courtesy of Rachael Ray