Preheat the oven to 375°F

Set a 14-inch paella pan or a 14-inch ovenproof sauté pan over medium-high heat and add 2 tablespoons of the olive oil. Season the shrimp with 2 teaspoons of the pimenton and ½ teaspoon of the salt. When the oil is hot, sear the shrimp, about 1 minute per side. Remove the shrimp from the pan and set aside. Add the remaining olive oil and the chorizo to the pan and sear, stirring occasionally, until the sausage is nicely browned, 2 to 3 minutes.

Add the onion, bell peppers, garlic, and saffron to the pan and sauté, stirring often, until lightly caramelized, 6 to 7 minutes. Add the rice to the pan and cook, stirring, for 3 minutes. Season with the remaining 1½ teaspoons salt and ½ teaspoon pimenton. Add the stock to the pan, bring to a boil, reduce the heat to a simmer, and cook, undisturbed, until the rice no longer soupy but some liquid remains, 5 to 6 minutes. Give the rice a quick stir, then add the clams, with the opening-edge facing up. Reduce the heat to low and continue to simmer for 5 minutes. Add the peas, mussels (again, opening-edge facing up), and shrimp to the pan and cook, undisturbed, for 10 minutes.

Transfer to the oven and cook until the clams and mussels have opened up, the liquid is mostly absorbed, and the rice is almost cooked through, about 10 minutes.

Remove the paella from the oven and cover with foil. Let it sit for another 10 minutes before serving. (The rice will continue to cook and absorb liquid as it sits.) Serve garnished with the chopped parsley.