To toast walnuts, place in a shallow baking pan.
Bake at 350*F (175*C) for 15 to 20 minutes or until walnuts are lightly browned, stirring occasionally.
Cool.
In large skillet, saute onions in butter until they become transparent.
Remove from heat and add chilies, crab, shrimp, walnuts, and olives.
Combine cheeses, reserving 1 1/2 cups of the cheese mixture for top of casserole.
Stir remaining cheese into seafood mixture.
In medium skillet, heat 1/4 inch oil.
Fry tortillas, one a time, just long enough to soften, about 30 seconds.
Drain on paper towels.
Fill each tortilla with seafood filling, roll up and place seam side down in a lightly greased 13 x 9 x 2-inch baking dish.
In medium saucepan, combine the half and half, sour cream, butter, oregano, salt and garlic powder, stirring constantly over medium heat until lukewarm and well blended.
Remove from heat and pour sauce over enchiladas.
Sprinkle enchiladas with reserved cheese.
Bake at 350*F (175*C) for 30 minutes or until bubbly.
Allow to set 5 minutes before serving.
