Fennel-Potato Soup with Smoked Salmon

INGREDIENTS

• 1 teaspoon fennel seeds
• 5 1/2 cups (or more) low-salt chicken broth
• 1 large leek (white and pale green parts only), halved lengthwise, thinly sliced crosswise
• 3 tablespoons butter
• 2 medium fennel bulbs, trimmed, chopped, fronds chopped and reserved
• 1 1/2 pounds russet potatoes (about 2 large), peeled, cut into 2-inch cubes
• 1 ounce smoked salmon, chopped