Pile the bread cubes into a very large bowl and set aside. Set a large sauté pan over medium heat and add the olive oil, half of the sausage, and the chopped turkey liver (if using). Cook, breaking up the sausage with a wooden spoon or spatula into 1/2- to 1-inch bits, until light brown, about 5 minutes. With a slotted spoon, transfer the sausage to the bowl of cubed bread; repeat with the remaining sausage. In the fat left in the pan, sauté the onions, celery, and garlic until the onions are translucent and just beginning to brown, 8 to 10 minutes. Stir in the thyme, sage, salt, and pepper, cook 1 minute, and then add the mixture to the cubed bread.
With the pan off the heat, carefully pour in the Marsala. Keep your face away from the pan as the wine will sputter. (It’s unlikely that Marsala will ignite, but if it does, just back off and let it burn for a few seconds until the alcohol has cooked off.) Set the pan over medium heat and bring the liquid to a boil, stirring to scrape up any flavorful bits in the pan. Boil for 2 minutes and then add the Marsala to the bread mixture; stir until well combined. Taste and add salt and pepper if needed.
You’ll have enough of the stuffing to fill a 12- to 14- pound turkey and a 9x13-inch baking dish. See the corresponding recipe for Stuffed Roast Turkey for instructions on how to prepare and stuff the turkey and how to bake the stuffing.