Scoop out melon with melon baller. Cut peel from oranges, remove white membrane, and slice crosswise. Cut grapes in half and remove seeds. Line a glass bowl with lettuce leaves; arrange melon balls, orange slices, grapes, and walnuts in layers on top of lettuce. Mix and blend well all ingredients for the dressing. Adjust seasonings. Pour dressing over fruit. Let salad ingredients marinate for 30 minutes. Toss salad just before serving.