Cut the squash in half lengthwise. Place, cut side up, in a casserole dish with ½ inch of water. Cover tightly with foil and bake until easily pierced with a fork, about 40 to 45 minutes. Or, if you don’t have a knife that’s sharp enough, just wrap the whole squash up in foil, place in a casserole dish, and bake until you can easily pierce with a fork. This will take 45 minutes or more depending on the size and density of the squash.
When the squash is just cool enough to handle, scrape it lengthwise with a fork into a bowl, to remove the spaghetti-like strands of flesh. Discard the seeds as you do so.
Combine the spaghetti squash strands with the remaining ingredients except for the nuts in a mixing bowl. Toss well and stuff back into the squash shells.
Return the filled squash shells to the baking dish and bake until well heated through, about 8 to 10 minutes.
Cut each squash half in half yet again to make 4 portions, and serve.