1. Preheat oven to 400° F. Heat oil in a large skillet over moderate heat. Add garlic, green onions, and chiles and cook until vegetables are soft but not browned. Stir in the tomatillo puree and cook mixture 10 minutes over low heat, adding a little water if the sauce starts to dry out. Add salt to taste.

2. Combine cilantro and shellfish in a bowl; stir in a small amount of the sauce. Moisten bottom of a shallow 8 by 13-inch baking dish with about 1/4 cup of the sauce.

3. Dip a tortilla into the simmering sauce until soft and pliable, about 30 seconds; transfer to prepared baking dish. Place 1/4 cup of the seafood across the center of tortilla and roll it into a cylinder. Repeat with remaining tortillas, fitting them snugly side by side. Pour remaining sauce over enchiladas, top each one with a spoonful of sour cream, and sprinkle cheese over the top. Bake 10 minutes and serve with the sauce from the baking pan.