To a saucepan over medium heat, add the oil. Sauté the onion, garlic, and chipotle powder with the kosher salt for about 5 minutes. Add the remaining ingredients and simmer over medium heat until thick and syrupy (the bubbles will get larger as the sauce reduces), about 30 to 45 minutes. Carefully transfer to a blender or food processor, and blend until smooth. Store in an air-tight container in the refrigerator for up to one week, or in the freezer for up to six months.