Cook potatoes and carrots in boiling salted water in large saucepan on medium-high heat 10 minutes or until fork-tender, adding peas during last 5 minutes of cooking. Drain well. Cool slightly.
Mix mayonnaise, crema, salt, thyme and pepper in medium bowl until well blended. Add celery and green onion to cooked vegetables; mix well. Add mayonnaise mixture; toss gently to coat. Cover.
Refrigerate until ready to serve.