In 4-quart saucepan, heat oil, onion and garlic over medium-low heat 5 to 10 minutes, stirring frequently, until onion is soft and translucent.
Add tomatoes; heat to boiling. Reduce heat; simmer 30 minutes, stirring occasionally. Remove from heat; add yogurt, Parmesan cheese, 2 tablespoons basil leaves, the salt and pepper. In blender, place soup mixture. Cover; puree until smooth.
Spread butter on 1 side of each bread slice. Arrange 3 slices bread, buttered side down, on work surface, and place 2 slices cheese and 1 teaspoon of the basil leaves on each bread slice. Top with remaining 3 bread slices, buttered sides up. In 10-inch nonstick skillet, cook 3 sandwiches at a time over medium heat, turning once, until golden brown and cheese just melts. Cut each sandwich into 6 squares. Place 2 sandwich squares onto each skewer.
To serve, spoon soup into small cups; place sandwich skewer on top of cup.