For the sauce: In a blender or food processor, combine the peppers with the stock, garlic, onions, cilantro and some salt and pepper, then puree. In a saucepot over medium heat, melt the butter, then whisk in the flour. Add the milk and cream and raise the heat a bit. Add the poblano puree and simmer to thicken and cook the sauce, 20 to 25 minutes.
For the casserole: Heat the EVOO in a large skillet over medium-high heat; add the butter and melt into the oil. Brown the mushrooms, about 10 minutes. Then add the corn and shallots, and cook to lightly brown the corn and soften the shallots, about 5 minutes more. Add the turkey and stock and heat through.
Char the tortillas over an open flame on a gas burner, or in dry skillet.
Layer half the turkey, mushrooms and corn in a casserole dish, and top with half the sauce, cheese and a shingled layer of 3 tortillas. Top the tortillas with the remaining turkey, mushrooms and corn, tortillas, sauce and cheese. Cool and store for a make-ahead meal, or bake to serve. Bake at 400 degrees F from room temperature until heated through and brown and bubbly.
To roast turkey breasts when it's not Thanksgiving time, preheat the oven to 325 degrees F. Rub the skin of the whole turkey breast with butter or oil, sprinkle with salt and pepper and cook until temperature reads 165 degrees F on thermometer (1 hour for boneless and 75 to 90 minutes for bone-in). Let rest covered with foil.