Preheat oven to 375 degrees F. Trim away tough stalks and bottom stem from fennel bulb; reserve leafy tops. Cut the bulb and tender stalks into 1/2-inch-thick slices.

Arrange fennel and onion in a 13x9x2-inch baking pan. Drizzle with olive oil and sprinkle with salt. Roast in the preheated oven for 25 minutes or until the vegetables are just tender but not brown.

Transfer roasted fennel and onion to a large saucepan. Add potato and broth; bring to boiling. Reduce heat and simmer, covered, 10 minutes or until potatoes are tender. Cool slightly.

Place one-third of fennel mixture in a blender container or food processor bowl. Blend or process until smooth. Repeat with remaining fennel mixture. Return all mixture to saucepan. Stir in half-and-half or evaporated milk, cumin, and grapefruit juice. Heat through. Season to taste with white pepper.

Meanwhile, place the fennel seeds in a small skillet over medium-high heat and toast about 3 minutes or until lightly browned and fragrant, stirring frequently.

To serve, ladle soup into individual bowls. Top each serving with a piece of fennel top, and sprinkle with toasted fennel seeds. Garnish with edible flowers, if desired. Makes 8 (1-cup) servings.

Prepare soup; cool slighlty. Cover and chill for up to 2 days. Reheat over low heat to serve.