Using cooking spray, coat a 9 x 13 x 2-inch baking dish. Place half of bread slices in bottom of dish. With a pastry brush, brush top of bread slices lightly with one tablespoon of melted butter. Place the spears from 1 can of asparagus crosswise over the top of bread slices; sprinkle with 1 cup of cheese.  Place remaining bread slices on top. Brush tops of the bread slices with remaining melted butter, and repeat process with asparagus spears and cheese.  In a bowl, beat together eggs, milk, onion, salt, dried mustard and pepper; mix well.  Pour over bread, pressing bread with the back of a wooden spoon to make sure milk mixture thoroughly saturates and covers the bread, asparagus and cheese.  Cover and refrigerate overnight. Bake, uncovered at 325ºF until a knife inserted in center of strata comes out clean, or about 45-55 minutes.