Heat oven to 400ºF. Line a half-sheet pan with parchment paper and set aside.
Cut 1 inch off the top of each acorn squash and scoop out the seeds. If necessary in order for the squash to sit upright, cut off a small portion of the bottom. Put 1 of the 4 pieces of butter in the cavity of each squash. Set squash on the prepared sheet pan and set aside.
Place an 11-inch saute pan over medium heat. Add 1 tablespoon of the olive oil and swirl the pan to coat. Add the ground pork and 1/2 teaspoon of the salt and cook until no longer pink, about 5 minutes. Remove the meat to a small bowl and set aside. Add the remaining olive oil, onion, celery, and carrot and sauté until they begin to soften, another 5 minutes. Deglaze the pan with the white wine and sherry vinegar and let simmer until almost evaporated, 2-3 minutes.
Return the pork to the pan along with the cooked rice, spinach, pine nuts, oregano, thyme, sage, remaining salt and the pepper. Stir to combine and heat thoroughly, 2 to 3 minutes. Remove from the heat and divide the mixture evenly among the squash, Top each squash with its lid, and bake until the squash is tender, about 1 hour. Serve immediately.