Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Cover pan and bring the water to a boil. Add asparagus, cover, and steam until just tender, 5 to 8 minutes.
Heat olive oil in a large skillet; cook and stir mushrooms and onion until the mushrooms have given off their liquid, 5 to 8 minutes. Stir in the asparagus, tossing vegetables just until hot. Remove from heat.
Mix saltine cracker crumbs, sharp Cheddar cheese, and black pepper together in a bowl; spread half the mixture into the bottom of the prepared baking dish.
Spoon the asparagus mixture over the crumb mixture.
Whisk the cream of mushroom soup and milk together in a bowl until smooth, and pour the soup mixture over the asparagus mixture.
Spread the remaining crumb-cheese mixture over the casserole and sprinkle with pecans.
Bake in the preheated oven until the casserole is bubbling, about 30 minutes.
