In a large bowl, combine water, salt and sugar. Add turkey breast. Let brine in the refrigerator for 1 hour.
With the rack in the middle position, preheat the oven to 180 °C (350 °F).
In a large 33 x 23-cm (13 x 9-inch) baking dish, combine squash, apple, garlic, sage and half the oil. Season with salt and pepper. Place slices around the baking dish. Set aside.
Drain and pat turkey breast dry.
In a non-stick skillet, brown meat on each side in remaining oil and place in the centre of the baking dish with vegetables. Roast for about 50 minutes or until a thermometer inserted in the centre of the breast reads 74 °C (165 °F). Turn squash slices and apples halfway through cooking. Cover and let rest for 10 minutes.
Meanwhile, in a non-stick skillet, brown pancetta slices on each side until crispy. Set aside on paper towel.
In the same skillet, melt butter. Add bread and pecans and sauté until bread is golden brown and crisp. Set aside in a bowl.
In a bowl, combine meat juices with broth to get 250 mL (1 cup) of liquid. Add broth, if needed.
In a saucepan, melt butter. Add flour and cook, stirring until mixture turns golden brown, about 3 minutes.
Add broth, Marsala, Worcestershire sauce and celery salt and bring to a boil, stirring with a whisk. Simmer for about 3 minutes over medium heat. Adjust seasoning.
Garnish four warm plates with a dash of mashed carrots. In each plate, layer a slice of squash, a slice of pancetta, a slice of apple and a second slice of squash. Sprinkle with crunchy stuffing. Finish with a slice of pancetta. Serve with turkey slices. Drizzle with Marsala sauce.
Marsala is an Italian fortified wine, which means that alcohol was added during fermentation. It has a slightly sweet taste. If you do not have Marsala, you can substitute the same amount of white wine or 60 ml (¼ cup) of brandy.