Heat butter and oil in medium saucepan on medium-high heat until butter is melted. Add onions; saute until translucent. Stir in stock; bring to boil. Reduce heat to medium-low. Stir in watercress and zucchini. Cook 8 to 10 min. or until zucchini is tender.
Whisk in RITZ Crackers, salt and pepper. Cook 5 min. Remove from heat. Puree with immersion blender or puree, in batches, in blender until smooth. Refrigerate until ready to use.
Combine yogurt and coriander. When ready to serve, portion 1 cup soup into each serving bowl. Top with 1 Tbsp. yogurt mixture and 1/2 Tbsp. basil.