1. Spread bread cubes out on a rimmed baking sheet (or two, if necessary) and toast in a 350 degree oven until dried out and slightly crisp (10-15 minutes). Set aside.
2. In a large skillet, melt butter over medium heat. Add onions and celery and cook, stirring occasionally, until very soft but not browned, about 7 minutes. Add apples and leeks and cook for a few minutes longer until apples are crisp tender. Add sage, thyme, and poultry seasoning, stir and cook for half a minute longer, then remove from heat.
3. Transfer onion/apple mixture to a very large bowl, add chopped bacon and bread cubes, and toss well. Add two cups of chicken stock, toss well, then add more stock, a little at a time until the stuffing is well-moistened, but not soggy. Taste, and add a bit of salt if desired.
4. Pour stuffing into greased casserole dishes (mine fit into one 9×12-inch dish, but you can divide it if necessary). Cover with foil and bake at 375 degrees for about 30 minutes. Uncover and bake an additional 20 minutes or until the top is golden brown.