Place a 10- to 12-inch heavy cast iron skillet in a cold oven; heat oven to 450 degrees F.
In a large bowl combine sweet peppers, fennel, olive oil, salt, and black pepper. Remove the hot skillet from the oven. Carefully pour the vegetables into the skillet. Roast, uncovered, for 12 minutes, stirring twice.
Carefully spoon roasted vegetables into a blender and add the 1 3/4 cups buttermilk and the water. Blend until smooth. Add liquor, if using, and blend again. Serve warm or at room temperature. Garnish with the 1/4 cup buttermilk and, if desired, roasted sweet peppers.