In a medium pot with a candy thermometer or deep-fry thermometer attached, heat the broth. When the liquid reaches 165 degrees F, with barely any bubbles reaching the surface, add the turkey. Adjust the heat, if necessary, to keep the temperature at 165 degrees F.
Using foil, cover the pot around the thermometer and poach the turkey until the internal temperature of the turkey reaches 150 degrees F, about 30 to 40 minutes. Turn off the heat and let the turkey rest in the poaching liquid for an additional 15 to 20 minutes.
Meanwhile, heat a large nonstick pan over medium heat. When hot, add the olive oil. Add the onions and celery and cook until the vegetables are soft, about 5 minutes. Season with salt and pepper to taste.
Add the poultry seasoning and bread. Stir to combine. Lower the heat to low and add the poaching liquid, 1/2 cup at a time (about 1 cup total), to the pan to moisten the bread.
Stir well, and then cover. Cook until the bread is moist and hot, about 5 minutes. Stir in the parsley and season with salt and pepper to taste. Set aside.
Remove the thermometer from the pot and bring the poaching liquid up to a simmer. In a small bowl, add the cornstarch. Whisk in about 1/2 cup of the poaching liquid, and then pour the mixture back into the pot. Turn up the heat so that the liquid boils.
Turn down the heat and simmer until the liquid thickens and reduces in volume by a third, about 10 minutes. If the liquid is too thin, mix more cornstarch with some liquid from the pot and stir it back into the simmering gravy. Season with salt and pepper to taste.
Slice the turkey breast as thinly as possible. Serve with the stuffing, gravy and cranberry sauce.