Step 1Using tongs, char the poblano peppers over an open gas flame until blackened in spots on all sides. (If using an electric stove, char peppers on a comal or dry skillet over high heat.) Be careful not to overcook them or else they'll turn limp and slimy -- you don't want a deep black burn but a sort of speckling of black and brown. Once peppers are charred, wrap them in a dish towel or paper bag. When cool, rub off the skin using your fingers or a paper towel. Try not to rinse them under water or you'll lose some of the charred flavor.
Step 2Once peeled, cut open each chile and scrape out seeds and veins. Cut the chiles into strips 2 inches long and one-half-inch wide.
Step 3In a large skillet heated over medium heat, add the oil. Stir in the onion and cook until soft and translucent, 3 to 5 minutes. Add the chiles and broth and bring to a simmer, then stir in the crema. Bring to a simmer again and stir in the cheese, stirring until the cheese is melted and creamy. This makes about 4 cups rajas con crema. Season to taste with salt and serve warm with tortillas.