Prepare Dinner as directed on package. Meanwhile, melt 2 tsp. butter in large saucepan on medium heat. Add onions; cook and stir 5 min. Stir in broth, water and tomatoes. Bring to boil; simmer on medium-low heat 10 min. or until onions are tender.
Blend tomato mixture, in small batches, in blender until smooth, returning each puréed batch to saucepan. Keep warm until ready to serve.
Fill bread slices with Singles to make 2 sandwiches. Spread outsides of sandwiches with remaining butter. Cook in skillet on medium heat 3 min. on each side or until Singles are melted and sandwiches are golden brown on both sides. Cut each sandwich into 12 pieces.
Spoon Dinner into 6 bowls; top with basil, soup and grilled cheese "croutons."