In a medium-sized saucepan, add the canola oil and heat over medium until hot. Add the onions and saute until softened and just turning brown, around 4-5 minutes. Add the Serrano peppers and continue to cook for an additional 2 minutes.
Add heavy cream and heat until gently simmering, then add the cheese, stirring occasionally as it melts to combine all the ingredients. As it starts to smooth out, add the milk and continue stirring until no more lumps of cheese remain. Gently stir in the tomatoes and cilantro. Serve immediately alongside a big bowl of chips, and keep leftovers in an airtight container in the fridge for up to 4 days.