Cook and stir reserved shrimp shells and 2 teaspoons olive oil in a saucepan over medium heat until shells are pink and fragrant, 1 to 2 minutes. Stir saffron into shells; add chicken broth, bring to a simmer, and cook until broth is a rusty brown and fragrant, about 20 minutes.
Strain saffron broth through a fine-mesh sieve; measure out 2 cups of broth, pour into a small saucepan, and place over low heat to keep broth hot.
Preheat oven to 425 degrees F (220 degrees C).
Heat 1 tablespoon olive oil in a large, oven-proof skillet over medium heat. Cook chorizo slices in hot oil until browned, about 2 minutes per side. Add onion to sausage; cook and stir until soft and slightly translucent, about 3 minutes. Reduce heat to medium-low.
Stir garlic into chorizo mixture; cook and stir until fragrant, about 1 minute. Add rice to skillet and stir to coat rice completely in oil; stir in peas.
Pat rice mixture evenly into the bottom of the skillet. Arrange shrimp in a single layer over the top of the rice. Lay pepper strips around and in-between shrimp; season with salt and cayenne pepper.
Increase heat to high. When rice begins to sizzle in the skillet, pour reserved 2 cups hot saffron broth over the shrimp; gently shake the skillet to distribute liquid.
Bake rice mixture in the preheated oven until rice is almost tender and still a bit wet, about 20 minutes.
Place skillet over medium-high and cook until rice is tender, liquid is absorbed, and rice caramelizes and crusts slightly on the bottom of the skillet, 3 to 5 minutes.