Cook macaroni according to package directions. Drain and set aside.
In large saucepan, cook butter, onion, asparagus, salt & pepper over medium heat until onion is tender. Stir in flour and simmer until mixture is bubbly.
Stir in milk and heat to boiling, stirring constantly. Add cheese and stir until melted. (Tip: don't over-cook or use too high of heat, otherwise the sauce will start to "break" - don't worry, it's not the end of the world if it does)
Add drained macaroni to cheese sauce and stir. Put in large baking pan, sprinkle with Parmesan cheese and bake uncovered at 375 degrees for 45 minutes (or until it's bubbly and top is lightly browned)
Serve topped with fresh tomato and Parmesan.
Recipe by Food. (Just Sayin') at http://www.foodjustsayin.com/2014/05/asparagus-mac-cheese/