Heat the oil in a heavy saucepan over medium-high heat. Add onion and cook until it begins to brown, about 7-10 minutes, stirring occasionally. Add chipotle paste and continue to cook for another minute until it becomes fragrant. Add the flour and cook for at least 2 minutes, stirring constantly. The flour will brown slightly.
Whisk the broth into the roux. Stir as the sauce thickens, remove from heat, add sour cream and milk. Season with salt and pepper.
Blend in blender until smooth. If your sauce is too thick, add more milk by a teaspoon at a time. Rewarm gently in the saucepan, but do not boil, as the sauce will break. Taste for salt and pepper.