Heat oil in a 9- or 10-inch cast iron skillet over medium heat until shimmering. Add the chorizo and cook, stirring occasionally, until the fat renders and it browns, about 5 minutes. Stir in the garlic and sauté until fragrant, about 30 seconds more.
Add the rice and cook, stirring occasionally, until lightly toasted, 2 to 3 minutes. Add the broth, tomatoes and their juices, salt, a few grinds of pepper, and saffron and bring to a simmer. Cover and cook without stirring until the rice is tender and most of the liquid is absorbed, 18 to 20 minutes.
Scatter the shrimp and peas over the rice, cover, and cook until the shrimp are pink and opaque, 3 to 4 minutes more. Taste and season with additional salt if needed. Serve immediately.