Cook pasta per box instructions.
In a large skillet, heat the olive oil on medium heat and saute the the onion, garlic and asparagus for about 5 minutes or until the asparagus is tender, but not over-cooked. The asparagus should be bright green and you should be able to pierce with a fork.
Remove the asparagus from the heat and stir in the lemon juice..
Combine the asparagus with the pasta.
Top with Parmesan when serving.