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.
In a large saute pan set over medium heat, brown the ground pork until no longer pink, 5 to 6 minutes. Remove the meat from the pan, add the olive oil, and saute the onion, celery, and carrot until they begin to soften, 7 to 10 minutes. Deglaze the pan with the white wine.
Return the pork to the pan along with the cooked rice, spinach, pine nuts, oregano, salt and pepper, to taste. Stirring constantly, heat mixture thoroughly, 2 to 3 minutes. Remove from the heat. 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.