The bread is cooked like a savory French toast, giving you a stuffing base that's both crisp and custardy. You can call it stuffing or you can call it dressing, but whatever you call it, it's one of my favorite parts of Thanksgiving dinner.
For the French toast batter: In a bowl, whisk buttermilk, heavy cream, eggs, and garam masala. Pour over bread cubes, toss, and let rest so the bread can absorb all of the liquid. For the stuffing: Peel, core, and evenly slice the apples. Melt 2 tablespoons butter in a skillet over high heat; add the apples, turn the heat to medium, and cook for 5 minutes. While the apples are cooking, core and thinly slice the fennel and onions so they are the same size. Season apples with a pinch of garam masala, a pinch of salt, then toss, remove from skillet and set aside.
Add 1 tablespoon butter to the same skillet and add the fennel and onions, along with the tarragon and a pinch of salt. Cook until just tender, 5 minutes. Remove from the skillet and set aside.
In the same skillet over medium heat, brown the sausage and break it up. After a minute or two, flip to brown on the other side. Remove and set aside. Over medium heat, melt 2 tablespoons of butter in the skillet and add half of the bread/French toast mixture in an even layer. Add a pinch of salt and continue cooking until the bread is brown and caramelized in parts; set the browned bread aside. Repeat with the remaining butter and bread/French toast mixture. Add all of the bread to a serving bowl along with the apples, fennel, onion, and sausage. Toss, then top with the tarragon and pistachios, and serve.