1. Spread bread cubes on baking sheet, and dry overnight. Or dry in 250°F oven 45 minutes to 1 hour, stirring occasionally. Cool, and transfer to large bowl.
2. Preheat oven to 350°F, and coat 13- x 9-inch baking dish with cooking spray. Thinly slice leeks, keeping white and green parts separated.
3. Heat 2 Tbs. oil in large skillet over medium-high heat. Add white leek slices, carrot, and celery. Cook 15 minutes, or until leeks are soft and carrots tender, stirring frequently. Add 1 Tbs. oil, and stir in green leek slices, garlic, and sage. Season with salt and pepper, if desired. Cook 7 minutes, or until garlic is fragrant and leeks are soft. Stir into bread cubes.
4. Whisk miso with 1/2 cup broth until smooth; gradually whisk in remaining broth. Pour over bread mixture, and stir until moist, but not soggy. Spread stuffing in prepared baking dish, and drizzle with remaining 1 Tbs. oil. Bake 35 to 40 minutes, or until top is browned and crisp. Let stand 15 minutes before serving.