In a medium saucepan cook onion in butter until tender.  Stir in tomatoes, chile peppers, and cumin.  Heat to boiling, reduce heat.  Simmer, uncoered for 10 minutes, stirring occasionally.

Toss shredded cheese with cornstarch.  Gradually add cheese mixture to saucepan, stirring until cheese is melted.  Gradually add the cream cheese, stirring until cheese is melted adn smooth.  heat through.  Serve with chips.