Prepare the poblanos. Roast the poblanos over an open flame or close up under a preheated broiler, turning until blistered and blackened all over. Let cool until handleable, then rub off the blackened skin, tear open and pull out the seed pod. Rinse the poblano briefly to get rid of stray bits of blackened skin and seeds. Cut into ¼-inch strips that are about 2 inches long.
Finish the dish. In a large (10-inch) skillet over medium-high, heat the oil. When hot, add the onion and cook, stirring regularly until lightly browned, 4 or 5 minutes, then add the garlic and stir continually for about 1 minute, until fragrant. Add the poblano strips, oregano and crema (¾ cup if not using add-ins.) Keep cooking until the crema has reduced enough to coat everything richly, about 3 minutes. Taste and season with salt, usually about 1 generous teaspoon. Serve with warm tortillas and cheese.
How to treat the add-ins: Scoop the greens into a large microwave-safe bowl. Splash in a couple tablespoons of water, cover and microwave on high until the greens are just tender, anywhere from 30 seconds for tender spinach to 2 minutes for tougher kale and lambs quarters. Cool until handleable, then squeeze out as much water as possible.
Whether you’re adding greens, chicken, or both, add to the skillet along with the poblanos. For one add-in, increase the crema to 1 ¼ cup. If you’re using both add-ins, increase the crema to 1 ½ cups.