Preheat the oven to 375°F.
Butterfly the turkey breast by slicing the meat horizontally, cutting almost to the other side. Open it like a book and place it between two sheets of plastic wrap. Pound with a meat mallet to a thickness of about 1/2 inch and season on both sides with salt and pepper.
Chop one onion and thinly slice the other onion into half-moons. In a large skillet, heat 1 Tbsp oil over medium heat and cook the chopped onion, stirring, until golden, about 8-10 minutes. Add the garlic and cook another minute. Add the bread, cranberries, toasted pecans, 2 Tbsp sage, and about 1/2 cup broth (stuffing should be moist, not wet). Cook over low heat for 2 to 3 minutes, then remove from the heat.
Spread the stuffing over one side of the turkey breast, leaving 2 inches uncovered on all sides. Roll up and secure tightly with kitchen twine.
Heat the remaining oil in a large Dutch oven over medium heat. Sear the stuffed turkey breast on all sides until lightly browned, 3 to 4 minutes per side. Sprinkle the sliced onion around the turkey, pour in 1 1/2 cups broth, cover tightly with a lid or aluminum foil and roast in the oven for 45-60 minutes, or until an instant-read thermometer registers 165 degrees in the thickest part. Remove from the oven, transfer to a cutting board and cover with aluminum foil.
In the roasting pan set on the stove over medium heat, combine apple cider, remaining broth (1/2 cup), vinegar, and 2 tsp sage. Bring to a boil, and cook until the liquid is reduced by one-third, about 10 minutes. Slowly add the cornstarch mixture, stirring constantly, and cook for 3 minutes more. Season with salt and pepper to taste.
Remove the twine from the turkey breast and cut it into slices. Serve with the gravy on the side.