Let the salad stand at room temperature at least 30 minutes before serving. This allows the dressing to penetrate the vegetables and tenderize them a bit for a less aggressive crunch.
Peel and section orange and grapefruit over a bowl; squeeze membranes to extract juice. Set sections aside; reserve 3 tablespoons juice. Discard membranes. Combine juice, oil, vinegar, and honey in a large bowl. Add fennel, radicchio, and onion; toss to coat. Cover and let stand 30 to 60 minutes. Top with orange and grapefruit sections; sprinkle evenly with salt.