In a Dutch oven or other large, heavy pot, melt butter over medium heat. Add onions and cook, stirring, until softened, about 10 minutes. Add garlic and cook, stirring, for 2 minutes. Add plum tomatoes and their juice, chicken broth and crushed tomatoes; bring to a boil; then simmer for 5 minutes. Add red peppers, Pernod (if using), thyme, salt and pepper; bring to a boil; then simmer for 5 minutes longer.
In a bowl, stir together sour cream and lemon peel. Cover and refrigerate.
Strain soup into a large bowl. Puree strained solids in batches in a blender or food processor, adding soup liquid to blend. Strain again over same bowl and discard solids. Wipe out Dutch oven, add soup and bring to a simmer.
Strain soup into a serving bowl, dollop sour cream on top and garnish with thyme.