Put the poblano peppers over an open flame to burn the skin, turning them with tongs to help flame all the corners and spots on the pepper. When all the skin is black, seal the peppers in a plastic bag so the steam will accumulate to help release the skin. Remove the charred skins using a napkin. Do not rinse the peppers; the flavor will be lost. Cut the peppers into strips, discarding the seeds, and set aside.
Char the jalapeno pepper in the same way, if using, but dice very small without peeling or de-seeding.
Heat the oil over medium-high heat in a wide pan. Fry the onions until translucent, and then stir in the tomatoes and fry until cooked through. Add the corn, cook for 1 minute, and then add the queso fresco and sour cream. Turn the heat down to medium and warm the mixture (don't let it boil or the cream might break), and then stir in the poblano peppers and jalapeno, if using, for 1 minute. If the poblano is too crunchy or raw, cook for a few minutes longer until soft.
Season with salt and serve on crunchy tostadas or in warm corn tortillas as tacos.
Before adding the jalapeno, remember that poblano peppers are usually mild but sometimes they can be spicy, so adding more heat might not be necessary.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.