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.
