Roast the Peppers:
Roast the poblano peppers on a hot skillet or under the broiler until the skin is blistered and blackened on all sides.
Place the roasted peppers in a plastic bag or covered bowl to steam, which will help the skins peel off more easily.

Prepare the Peppers:
Once cool enough to handle, peel the skin off the peppers. Open them to remove the seeds and veins, then slice into thin strips ("rajas").

Cook the Onion and Garlic:
In a large pan, heat the oil over medium heat. Sauté the onions until they become translucent, then add the garlic and cook for another minute.

Combine Ingredients:
Add the poblano strips (and corn if using) to the pan with the onions and garlic. Cook for a few minutes until everything is well combined and the corn is tender.

Add Crema:
Reduce the heat and stir in the Mexican crema (and milk if you want a thinner sauce). If using cheese, add it at this point and stir until melted. Season with cumin, salt, and pepper.