Peel and section grapefruit and oranges over a bowl; squeeze membranes to extract juice. Set sections aside; reserve 1/4 cup juice. Discard membranes.
Heat oil in a large Dutch oven over medium heat. Add onion, and sauté 30 seconds. Stir in reserved juice, vinegar, honey, and salt. Add fruit and spinach; cook 1 minute or until spinach begins to wilt, tossing gently. Arrange spinach mixture on individual salad plates; top with croutons.