In a medium pot over medium-high heat, combine onions, garlic, 1 cup corn kernels, 2 cans of the chiles, chicken stock, and Mexican seasoning. Bring to a boil. Reduce heat and simmer for 10 minutes.
Meanwhile, melt butter in medium skillet over medium-high heat. Add remaining corn and green chiles. Add mushrooms and saute for 10 minutes, or until water from mushrooms is released and evaporated; set aside.
Transfer chile mixture to a blender. Puree until smooth, working in batches if necessary. *(TIP: Cover blender with dish towel and pulse.) Return puree to skillet and add sauteed vegetables. Bring to a boil. Reduce to low heat and stir in Mexican crema. Season, to taste, with salt and pepper.
Serve soup hot garnished with cilantro and cotija cheese.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.