Mist 12 cups of a standard size, nonstick muffin tin with cooking spray. Set aside.
Spray a small, nonstick skillet with cooking spray and place it over medium heat. Toss in the onion and garlic. Cook, mixing occasionally, for about 5 minutes, or until almost tender. Taking the pan off the heat, place to the side for later.
In a large measuring cup, blend the milk, egg whites, mustard, salt, and pepper. Whisk to mix well. Set aside.
With a 3 ¾ round cookie cutter, cut circles out of each tortilla. Fit each into one of the 12 cups in the prepared tin, taking care not to tear the circles.
Distribute the cheese, asparagus, and reserved onion combination between the cups.
Add the egg white mixture evenly into each cup until it is three-quarters filled. (If a small amount is left over, get rid of it.)
Bake for 20-22 minutes or until the egg-white mixture has set and the tortillas are evenly crisped around the edges. Serve right away.