Crumble chorizo and brown at medium heat for about 5 min. Transfer to a paper towel lined plate. In the same pan, add the butter and onions. Cook for 5 min. Transfer to a bowl.
Mix in other ingredients (except stock) and fold in chorizo. Add to prepared dish and pour over stock. Bake at 400°, uncovered, until the top is golden brown, about 20-25 minutes.