Preheat oven to 400 degrees. Heat a large, heavy ovenproof skillet (preferably cast iron) over medium-high heat. Swirl in oil. Cook chorizo, stirring often, until edges are browned, about 2 minutes. Stir in onion and garlic. Cook until translucent, about 3 minutes.
Stir in rice to coat, then wine. Bring to a boil. Cook until skillet is almost dry. Add stock and water. Bring to a boil.
Transfer skillet to oven. Bake for 10 minutes. Add clams. Bake until clams open and rice absorbs all the liquid, about 10 minutes. Let stand, covered, for 10 minutes before serving.
Sausage-and-Broccoli Rabe Variation: Replace chorizo with 8 ounces Italian sausage, casings removed, broken into pieces. Cook until opaque, about 3 minutes. Replace water with 3/4 cup chicken stock. Replace clams with 6 ounces broccoli rabe, cut into 2-inch pieces (5 1/2 cups) and tossed with 1 tablespoon water and 1/4 teaspoon coarse salt. Stir once removed from oven.