Heat the oil in a medium saucepan over medium-high heat. Add the onion and jalapeño to the pan and cook, stirring occasionally, until tender, about 5 minutes. Reduce the heat to medium-low. Add both of the shredded cheeses and ¼ cup of the milk. Whisk or stir until completely melted. Mix in the tomato and cilantro, and add additional milk gradually as necessary to achieve desired consistency. Transfer to a warmed serving bowl and serve immediately.