Place the butter in a saute pan set over medium heat. Once it melts, add the slivered onion and allow it to sweat for about 12 minutes, until translucent and soft. Increase the heat to medium-high, add the tomatoes and cook for about 2 minutes. Stir in the chile poblano rajas or strips, corn, salt and black pepper and cook for about 3 more minutes. Add the cream and queso fresco and continue cooking, stirring from time to time, until the sauce thickens a bit and seasons, for 2 to 3 minutes.