Preheat oven to 350°. Bake walnuts in a single layer in a shallow pan 6 to 8 minutes or until toasted and fragrant, stirring halfway through.
Remove and discard outer leaves of endive. Rinse endive with cold water, and pat dry. Cut each endive head diagonally into 1/4-inch-thick slices, and place in a serving bowl. Add walnuts, parsley leaves, and desired amount of dressing; gently toss to coat. Top with grapefruit. Serve with any remaining dressing.
Note: Total time includes vinaigrette preparation.