Preheat the oven to 450°F. Measure the water, apple juice, butter, onion, celery, thyme, sage, fennel seeds, and the meatless sausage into a large pot. Sprinkle with salt and pepper, then stir to combine. Cover and set over a high heat. Bring to a boil, then reduce the heat to medium and cook at a medium boil, stirring occasionally for about 5 minutes, until the celery just softens. Remove from the heat. Stir in the cut bread, applesauce, and apple, mixing well to combine. Taste for seasonings.
Divide the stuffing evenly among the squash halves, mounding as needed. Arrange the stuffed squash in a roasting pan (or two if needed). Pour water into the pan, around (not over) the squash, to a depth of 1inch. Spray the non-shiny side of a large sheet of foil (if the shiny side is down, the stuffing will get too brown) with cooking oil spray, then cover the pan tightly, tenting as needed to avoid pressing on the stuffing. Bake for about 1½ hours, until the squash is tender when pierced with a fork.