Preheat oven to 375° F.
Fry onions in butter until tender. Mix with remaining casserole ingredients, turn into baking dish and bake until set, about 50 minutes.
Sauce: Melt butter in saucepan. Stir in flour until well blended. Blend in milk and cook, stirring constantly, until thickened. Stir in mustard and salt until well mixed; fold in chives, pimento and mushrooms. Heat through, serve over asparagus custard.Servings: 4