Preheat oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.
Arrange asparagus spears in the baking dish.
Mix mushroom soup, eggs, mushrooms, water chestnuts, and pimentos in a bowl; pour over the asparagus.
Bake in preheated oven for 20 minutes. Top casserole with French-fried onions and continue baking until the onions begin to darken, about 10 minutes more.