For best results, use fresh, plump asparagus spears. Avoid using very thin asparagus, as it can become overcooked and limp. If you prefer a stronger onion flavor, you can add a pinch of garlic powder to the onion soup mix. Feel free to experiment with different types of cheese. Provolone, Gruyere, or even a sprinkle of crumbled feta would be delicious. To add a touch of heat, sprinkle a pinch of red pepper flakes over the asparagus before baking. This dish is best served immediately while the cheese is still melted and gooey.