To prepare leeks, arrange leek halves in an 8-inch baking dish; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add broth and next 3 ingredients (through 1 thyme sprig). Cut 1 (8-inch) square of parchment paper; lightly coat with cooking spray. Place parchment over leek mixture, coated side down. Bake at 325° for 50 minutes or until leeks are tender. Let stand 5 minutes; drain cooking liquid through a sieve over a bowl, reserving solids.
Place cooking liquid in a small, heavy saucepan; bring to a boil. Cook until reduced to 1/4 cup (about 8 minutes). Chop cooked carrot; set aside. Coarsely chop cooked garlic; set aside.
To prepare dressing, heat oil in a small skillet over medium-high heat. Add pancetta to pan; sauté 5 minutes or until crisp. Stir in garlic and chopped leek; sauté 2 minutes, stirring occasionally. Sprinkle with sugar; sauté 1 minute or until sugar dissolves. Stir in vinegar; simmer 2 minutes. Add braising liquid, 1/8 teaspoon pepper, and dash of salt; simmer 2 minutes or until slightly thick. Remove from heat. Arrange leek halves in a serving dish; sprinkle with carrot. Drizzle pancetta mixture over leek halves. Garnish with thyme sprigs, if desired.