Peel the oranges and grapefruit, separate into segments, then remove any white pith and pips. Cut the citrus segments into bite-sized pieces. Remove the skin from the onion and slice into thin half rings. Place the sliced onions in a salad bowl with the orange and grapefruit pieces.
Juice the lemon into a small bowl, add the red wine vinegar, olive oil and capers, whisk, and drizzle over the citrus fruit and onions. Salt and pepper to taste. Toss and arrange on individual salad plates or serve directly from the bowl.
Enjoy!