I found this recipe, years ago, in an issue of Quick Cooking (a Taste of Home spin-off that's been renamed). It's one of the first recipes that I kept, and, having resurrected it, I can see why -- it's easy and delicious!
In a 1-½ qt. microwave-safe dish, combine chilies, onion, and butter. Cover and microwave on high for 1-2 minutes or until butter is melted. Stir in flour until smooth. Gradually stir in milk until blended. Cook, uncovered, on high for 1 - 2 minutes or until thickened. Add cheeses and tomato.
Microwave, uncovered, at 70% power for 2 - 3 minutes or until cheese is melted. Serve with tortilla chips.