Heat the oil in a large skillet over medium heat. Add the onions and jalapeno with a small pinch of salt. Saute for about 6 minutes, until soft and fragrant. Add the cheeses and milk, stirring constantly until cheeses are melted.
Add the canned tomatoes, spinach and cilantro, and stir well to combine.
Transfer to a serving dish and serve with tortilla chips.
Reheat in the microwave or serve in a small slow cooker to keep warm for a longer period of time.