Roast the poblano peppers under a high oven broiler for about 5 minutes on each side until their skin is blackened and blistered. Cover them loosely to let them steam for about 5 minutes, which helps loosen the skin so you can pull it off easily. After that, remove stems and seeds and slice the peppers into strips.
Heat butter in a large skillet over medium-high heat. Add the sliced onion, cooking it until it becomes soft and translucent (about 5 minutes). Stir in the minced garlic for about 30 seconds, making sure it doesn’t burn but becomes fragrant.
Add the roasted pepper strips to the skillet, then pour in the Mexican crema and sprinkle in kosher salt. Stir together and let it cook for about 2 minutes. Finally, fold in the shredded Oaxaca cheese, mixing until it melts completely and the sauce becomes creamy and smooth. Remove from heat and serve immediately.