Preheat oven to 350°F (180°C). Combine brown sugar and half the salt and half the butter in a small bowl. Place acorn squash halves cut side up on a parchment paper-lined baking sheet. Brush the butter mixture inside squash. Tent squash with foil and roast for 1 hour or until squash is tender when pricked with a fork.
Meanwhile, heat olive oil in a medium saucepan and sauté onion for 2 min. over medium heat. Stir in garlic, walnuts, cranberries, rice, half the thyme, the remaining salt, and pepper and sauté for another minute.
Add water and bring to a simmer. Cover and cook on low for 20 to 25 min. or until water is absorbed and rice is tender. Remove cooked squash from oven and stuff with cooked rice mixture. Drizzle each half with the remaining butter, and sprinkle with thyme. Tent with foil again and return to oven for 5 min., then serve.