Steam whole unpeeled potatoes until soft when pierced with a fork, about 30 to 40 minutes. Cool and cube. Place in a large bowl and toss gently with tomatoes, onions and celery. In a small bowl, combine mayonnaise, yogurt and 1/4 cup pesto. Pour dressing over potato mixture and toss gently to combine. Chill before serving. PESTO: In food processor, place 1/4 cup nuts, 3 garlic cloves, 2 cups packed fresh basil leaves and 1/4 cup chopped fresh parsley; process until finely ground. With motor running, gradually add 2/3 cup olive oil until pesto is the consistency of mayonnaise. Add freshly ground pepper to taste. Transfer to a small jar and pour a thin film of olive oil over surface of pesto. Cover tightly and store in refrigerator. From "Delitefully HealthMark," by Susan Stevens, M.A., R.D.; SWS Publishing. Printed in the July 2, 1992 issue of the Los Angeles Daily News.