Add the smaller amount of árbol chiles for less heat. If you cannot find the dried chiles, you may substitute 3 canned chipotle peppers in adobo sauce, but remove the seeds.
Combine in a medium saucepan:
Bring to a boil, cover, and simmer until the onion and chiles are softened, about 15 minutes. Puree with an immersion blender (or in a food processor or regular blender) until smooth. Season to taste with:
Set aside. Heat in a large skillet over medium-high heat until hot:
Add the sauce and:
Simmer the shrimp for a few minutes, until firm and pink. Serve in all its saucy glory with: