Preheat the oven to 400 degrees . Reserve 4 strips of roasted red pepper and set aside. In a medium pot, place 4 tablespoons melted butter over medium heat. Add the leek and chopped fennel bulb and cook until lightly golden, about 10 minutes.
Add the remaining roasted red peppers, the potato, broth, 2 teaspoons salt and 1/2 teaspoon pepper and bring to a boil. Lower the heat and simmer, partially covered, until the potato is tender, about 10 minutes.
Meanwhile, place the pitas on a baking sheet and drizzle with the remaining 2 tablespoons melted butter. Sprinkle with some of the fennel fronds and salt; toast in the oven, 6 to 8 minutes. Set aside.
Using an immersion or standing blender, working in batches if necessary, blend the potato mixture until smooth. If desired, place a sieve over a large pot and strain the soup. Stir in the heavy cream and season with salt and pepper.
Serve the soup hot or cold with the pita chips. Dollop with sour cream, if desired, and top with the reserved roasted red pepper strips and more fennel fronds.