Using a food processor or blender, combine cream and chiles, adding chipotles a little at a time to taste, and pulse until smooth. Set aside.
Melt butter in a large heavy skillet over medium heat. Saute the green onion, garlic, and salt until soft, about 5 minutes. Add the sliced mushrooms and cook covered, about 3 minutes. Add the tomatoes and bring to a simmer. Reduce heat and cook, an additional 5 minutes. Add shrimp and simmer, covered until they begin to turn pink, about 2-3 minutes. Add broth if additional liquid is needed. Add the chipotle cream and heat until cooked through, an additional 3-5 minutes. Season to taste.
Serve with warm corn tortillas, over rice, or with avocado.
To make broth, combine reserved shells with 2 cups of water and 1/2 teaspoon of salt and bring to a simmer. Continue to simmer over medium heat for 30 minutes. Strain and use broth as needed to add liquid to the dish or thin the cream sauce to desired consistency.