Bake pastry shells according to package directions. Meanwhile, place butter in a 2-qt. microwave-safe dish. Cover and microwave on high for 20-30 seconds or until melted. Add the mushrooms, onions and pimientos. Cover and cook on high for 2-3 minutes or until vegetables are crisp-tender.
Combine flour and milk until smooth; gradually stir into the vegetable mixture. Microwave, uncovered, on high for 2-3 minutes or until thickened, stirring often. Add the cheese, crab, salt and cayenne.
Cook, covered, on high for 30-60 seconds or until the cheese is melted. Spoon into the pastry shells; sprinkle with parsley.