Fennel Soup with Orange Crème Fraîche and Croutons

Preheat oven to 375°F with rack in middle. Wash leek , then cook with garlic, fennel seeds, and 1 teaspoon salt in 3 tablespoons oil in a heavy medium pot over medium-low heat, stirring occasionally, until softened, about 8 minutes. Meanwhile, peel potato and cut into 1/2-inch pieces. Add potato, fennel, and water to leek mixture and simmer, covered, stirring occasionally, until vegetables are very tender, 15 to 20 minutes. Toss bread cubes with remaining 2 tablespoons oil in a shallow baking pan. Bake, stirring once or twice, until crisp and golden brown, 10 to 15 minutes. Purée soup in batches in a blender until smooth (use caution when blending hot liquids). Return to pot and season with salt and pepper. Chop 1 tablespoon fennel fronds and whisk together with crème fraîche and zest. Serve soup with orange crème fraîche and croutons.