Preheat the oven to 425°F and line a baking sheet with parchment paper. Scoop out and discard the seeds from the squash. Place the squash halves on the baking sheet, and drizzle them with olive oil and pinches of salt and pepper. Roast cut-side up for 40 minutes, or until tender.
While the squash roasts, cut the tempeh into ½-inch cubes, place in a steamer basket, and set over a pot filled with 1 inch of water. Bring the water to a simmer, cover the pot, and steam for 10 minutes. Remove, drain any excess water, and use your hands to crumble the tempeh.
Heat the olive oil in a large skillet over medium heat. Add the onion, ½ teaspoon salt, and several grinds of black pepper and cook 5 minutes. Add the mushrooms and cook, stirring, until soft, about 8 minutes. Stir in the crumbled tempeh, garlic, walnuts, tamari, apple cider vinegar, rosemary, and sage and cook 2 to 3 minutes more, adding ¼ cup water as the pan gets dry. Stir in the cranberries and season to taste. Scoop the filling into the roasted acorn squash halves and garnish with the parsley and pomegranates.