Preheat oven to 250°. Remove root, tough outer leaves, and tops from leeks. Cut in half lengthwise. Slice leeks; rinse well, and drain. Combine leeks and next 4 ingredients on a large rimmed baking sheet, tossing to coat with oil. Sprinkle onion mixture with 1 tsp. salt and ½ tsp. pepper. Bake, uncovered, at 250° for 1 hour, stirring every 15 minutes.

Cook bacon in a Dutch oven until crisp; drain, reserving bacon and 1 Tbsp. drippings in pan. Add onion mixture to bacon mixture. Add broth. Bring to a boil; reduce heat, cover, and simmer 30 minutes or until onions are soft. Remove from heat, and let cool slightly.

Process soup, in batches, in a blender until smooth. Pour each batch of pureed soup into a bowl. Return soup to pan; stir in cream, ½ tsp. salt, and remaining ½ tsp. pepper. Keep warm.

Preheat oven to 350°. Place bread cubes in a large bowl. Drizzle bread cubes with butter, and sprinkle with remaining ½ tsp. salt; toss well. Spread bread cubes in a single layer on a large baking sheet. Bake at 350° for 12 minutes or until crisp, turning with a wide spatula after 6 minutes.

Ladle soup into bowls, and sprinkle with croutons. Serve immediately.