In a blender or food processor, combine peppers with stock, onions, garlic, cumin, herb blend, cilantro, salt and pepper; purée.
In a saucepot over medium heat, melt butter and whisk in flour. Add milk and cream, and raise heat a bit. Add poblano purée and simmer to thicken and cook sauce, 20-25 minutes.
Preheat oven to 375ºF.
While the sauce is cooking, place a large skillet over medium-high heat and add the EVOO (3 turns of a pan). Season chicken breasts with salt and pepper and brown 3-4 minutes on each side then transfer to casserole dish. Spread poblano sauce over the top, cover with cheese and bake 15-20 minutes, depending on size of chicken pieces -- dish should be brown and bubbly on top.