Preheat oven to 350 degrees. Combine first 3 ingr in saucepan, and heat over med heat stiring once in a while. If using Sour cream add to this mixture.
While step one is heating cook onion and meat in large skilet till brown, then drain grease. Combine soup mixture, and meat together in skilet. Continue to heat over low heat.
In a geased cassarole or baking dish put shells along the bottom of pan, (they may overlap a little this is ok)and along the sides of the dish(you may have to tear the shells in half. Add skilet mixture to cassarole dish and top with chesse.
Heat in oven till chesse is melted, and cassarole is heated thru. This should take about 10 mins. If you want to you can cover with foil before putting in oven. When you remove from oven let stand for a minute to make easier to serve.