Preheat the outside grill or inside on the stove, preheat a grill pan.
With 1 1/2 Tablespoons of olive oil, rub or brush the oil on the corn, onion rings and the peppers. Grill the corn and onion rings until grill marks form on all sides and the veggies have softened. Grill the poblano pepper until it is blackened on all sides and softened.
Transfer the peppers to a large bowl and cover with plastic wrap (or put them in a large ziploc bag and close) and let steam for 10 minutes.
While the peppers are steaming, cut the corn off the cob and roughly chop the onions.
Peel off with your hands or the dull side of a knife as much of the charred skin as you can easily get off of the poblanos. Halve them lengthwise and discard the seeds and stems.
Chop the peppers and add to the corn and onion mixture.
In a large skillet on medium heat, add the oil. Add the garlic and cumin and cook just until fragrant, about 30 seconds.
Add pepper, corn and onion mixture. Season with salt and pepper. Cook for 3 to 5 minutes to continue to soften the onion.
Remove from heat and fold in the tomatoes (if using), Crema Mexicana, lime juice, cilantro and half of the queso fresco.
Taste and adjust seasoning with additional salt if needed.
Serve with warm tortillas.