Roast: Preheat oven to 400°F (204°C). Cut squash in half vertically (from north to south), and scoop out the seeds. Brush the insides with oil and sprinkle with salt and pepper. Set halves cut side up on a baking sheet, then bake for 45 to 60 minutes, until fork tender.
Veggies: Heat oil in a large pot over medium heat. Add carrot, celery, onion, and garlic. Cook until fragrant and veggies are brightly colored, about 5 minutes.
Rice: Add rice and broth, cover, and let simmer until rice is cooked and broth is absorbed, about 15 minutes, or until tender (will depend on your rice). Stir in feta, cranberries, apple, rosemary, sage, salt, and pepper. Cover and set aside.
Assemble: When squash is fork-tender, scoop flesh from the inside, being careful to not puncture the skin and leaving a thin layer on the outside (hold it with some paper towels if it’s too hot to handle). Stir the scooped out flesh in with the rice mixture, then stuff the mixture into each half of the acorn squash.
Bake: Return stuffed squash to the oven and bake for another 15 minutes, until slightly crispy and warmed through.