Saute onions and chilies in 1 tablespoons butter until onions soft and transparent.
Combine cheeses and set aside 1 1/2 cups for top of casserole.
Stir remaining cheese into onion mixture and add the crabs and shrimp.
Soften tortillas in microwave. Fill each tortilla with seafood mixture. Roll up and place seam side down onto greased 9 x 13 pan.
Can be refrigerated or frozen at this point.
In saucepan, combine half and half, sour cream, melted butter, oregan, and garlic. Stir till warm and blended. Pour over enchiladas. Top with remaining shredded cheese. Bake at 350 degrees for 30 minutes or till bubbly.
Options: can add cooked white rice to filling or serve on the side. can substitute shredded chicken for seafood.