Orange and Onion Salad

2 oranges
1 diced avocado
1 grapefruit
1 small red onion, thinly sliced and separated into rings
1/3 cup yogurt
2 Tbs chopped walnuts
Salad greens or spinach leaves
salt and pepper to taste

Peel and section the fruit then cut each section in two or three pieces and put in a bowl. Add all the juice from the fruit to the onion then add the avocado and yogurt. Toss and chill. Sprinkle with nuts, salt and pepper and serve on salad greens.