Roast the poblanos. Place the poblano peppers directly over a gas flame, on a hot comal, or underneath the broiler. Turn them regularly until the skins are blistered and blackened on all sides.
Steam the peppers. Transfer the roasted poblanos to a bowl and cover with a clean kitchen towel or plate. Let them rest for about 10 minutes. The trapped steam will help loosen the charred skins.
Peel and clean the poblanos. Once they're cool enough to handle, gently rub away the blistered skins. Cut off the stems, open the peppers, and remove the seeds and large veins.
Cut the rajas. Slice the roasted poblanos lengthwise into strips about 1/2 inch wide.
Cook the onions. Heat the oil in a large skillet over medium heat. Add the sliced onion and cook for about 5 to 7 minutes, stirring occasionally, until softened and beginning to turn golden around the edges.
Add the garlic. Stir in the minced garlic and cook for about 30 seconds, just until fragrant.
Add the poblanos and corn. Add the poblano strips and corn to the skillet. Cook for another 3 to 4 minutes, stirring gently, until everything is heated through.
Lower the heat. Reduce the heat to medium-low before adding the crema. This helps prevent the sauce from separating.
Add the crema. Pour in the Mexican crema and gently stir until the poblanos, onions, and corn are evenly coated.
Add the cheese. Sprinkle in the Oaxaca cheese and cook gently, stirring occasionally, until the cheese melts into the sauce.
Season. Add the salt and black pepper. Taste and adjust as needed.
Serve. Spoon the rajas con crema into bowls or directly into warm corn tortillas and serve immediately.