Make-Ahead Paella Casserole

1. Preheat the oven to 200 degrees Celsius. Butter a casserole dish with 2 tablespoons butter.
2. Heat 2 tablespoons butter in a medium saucepan over medium heat. Add the pasta and toast until golden. Stir in the rice and saffron. Add 3 cups stock and bring to boil. Reduce the heat to a simmer, cover and cook until al dente, about 17 minutes. Add an extra 1/2 cup water if the liquid evaporates before the rice is tender.
3. Meanwhile, season some flour with the paprika in a shallow dish. Sprinkle the chicken with salt and pepper, then dredge in the flour and shake off the excess. Heat 3 tablespoons olive oil in a large skillet or Dutch oven over medium-high heat. Add the chicken and cook until browned and crisp on both sides, 12 to 15 minutes. Transfer the chicken to a paper towel-lined plate to rest.
4. To roast bell peppers, arrange them on a baking sheet and broil, turning occasionally, until the skins are blackened. Transfer the peppers to a bowl, cover and let cool. Peel and seed the peppers.
5. Add 1 tablespoon olive oil to the skillet, then add the chorizo and cook until the fat begins to render, about 2 minutes. Add the garlic and onions and cook until soft, about 5 minutes. Deglaze with a splash of sherry, stirring and scraping up any browned bits from the bottom of the skillet with a wooden spoon. Stir in the peas, thyme and roasted peppers and immediately remove from the heat.
6. Slice the chicken and combine with the rice mixture and chorizo-pepper mixture. Transfer to the prepared casserole dish. Bake until heated through and the bottom is crisp, 30 to 40 minutes. Add a splash of stock, about 1/2 cup, if the top of the casserole dries out too much.
7. Meanwhile, cover the fish with about 1/2 cup kosher salt and let stand for 10 minutes. Rinse and pat dry. Season some flour with seafood seasoning in a small dish. Lightly dredge the fish in the flour and shake off the excess. Heat the remaining 1 tablespoon olive oil in a skillet over medium-high heat. Add the fish and cook until firm and opaque, 5 to 6 minutes, turning once. Transfer the fish to a plate. Add the prawn to the skillet and cook until pink, 4 to 5 minutes. Add the lemon juice and 1/2 cup sherry, then swirl in the remaining 1 tablespoon butter until melted. Add the parsley.
8. Flake the fish into pieces. Top the casserole with the fish, shrimp and sherry sauce. Divide among plates.

The casserole can be covered and refrigerated before baking for a make-ahead meal.