Peel shrimp and lightly saute in half the butter, set aside.
Saute the onions in the rest of the butter and set aside.
Pour some of the enchilada sauce on a deep plate or shallow bowl.
Dip both sides of a tortilla in the sauce, place on another plate, and fill with a bit of shrimp, cheese, and cilantro.
Roll into a log shape and place in an oblong glass dish that has a bit of sauce on the bottom.
Repeat with remaining tortillas.
Pour remaining sauce over the finished dish and sprinkle with cheese and olives.
Cover with foil and bake at 350 degrees F. for 40 minutes or until heated through and bubbly.
For my personal use I find the quality of the canned green enchilada sauce available retail just fine. If you are more of a purist, then feel free to prepare a sauce with fresh green chili peppers. Please note: how much sauce you need will depend on how wet you like your finished product. I use 3 cups.