In a heavy bottom pot, add 1 T butter, the onion, and the poblano peppers (minus the reserved portion).
Cook 4-5 minutes over medium heat, until the onions are slightly translucent.
In a small bowl, mix 1/4 cup chicken broth with 1 T cornstarch, and set aside.
Add 1/2 cup chicken broth to the onion and poblano mixture, and mix.
Place the warm onion, poblano, broth mixture into a blender with the cilantro and buttermilk. Blend until smooth.
Return the mixture to the pot and add the remaining 1/2 cup broth, the cornstarch mixture and the reserved poblano peppers.
Bring to a simmer over medium heat, stirring constantly.
Cook until gravy thickens and then season with salt and white pepper, according to personal preference.
Enjoy warm!