Preheat oven to 400 degrees Fahrenheit. Halve the acorn squash carefully, and scoop out the seeds (save them for roasting, yum!) and place the empty acorn squash halves on a sheet pan, cut-side up. Roast the squash for about 40 to 50 minutes until it is fork tender.
Meanwhile, in a large cast iron skillet brown the ground turkey over medium heat, cook for 8-10 minutes until the ground turkey shows no pink. Remove ground turkey using a slotted spoon, and set aside on a seperate plate. Leave the turkey drippings in the pan for the next step.
To the skillet add the olive oil, carrots, celery, onion, garlic, garlic salt, ground sage, and black pepper. Saute on low heat for about 7 or 8 minutes until the veggies have begin to soften. Add the cooked turkey to the pot, and cook over low heat until mixture is well-combined and hot.
Remove acorn squash from the oven, and carefully fill each squash with 1/4 of the cooked turkey mixture. Place the stuffed squash back in the oven for 15 minutes to heat.
Remove from oven and top with nutritional yeast (if desired), salt and a little extra ground black pepper. Mangia!