In a large, heavy saucepan, melt 3 tablespoons butter over medium-high heat. Add 2 coarsely chopped 12-ounce fennel bulbs, 1 sliced leek, 2 sliced cloves garlic and 1 teaspoon fennel seeds. Cook, stirring occasionally, until lightly browned, 8 to 10 minutes. Add 1/4 cup pernod and cook for 30 seconds. Add 4 cups chicken broth and season with salt and pepper. Simmer until the fennel is very tender, 10 to 12 minutes. Using a food processor and working in batches, puree the soup. Garnish with fennel fronds.