Heat the vegetable oil in a medium saucepan over medium heat until hot, about 1 minute.
Add the onion and Baby Bam and cook, stirring, until soft, about 5 minutes.
Stir in the tomato, if using, and garlic and cook, stirring, for another 2 minutes.
Reduce the heat to medium-low and add the cheeses and chiles. Cook, stirring constantly, until the cheese melts, about 2 minutes.
Stir in the sour cream, white pepper, and salt and serve immediately with tortilla chips.
YIELD: 3 cups, serving 6 to 8