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 a sheet pan lined with parchment paper. Set aside.
In a large saute pan over medium heat, brown the ground pork until no longer pink. Remove the meat from the pan, add the olive oil and saute the onion, celery, and carrot until they begin to soften, approximately 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 and salt and pepper, to taste. Stirring constantly, heat mixture thoroughly, approximately 2 to 3 minutes. Remove from the heat. Divide the mixture evenly among the squash, top each squash with its lid and bake for 1 hour or until the squash is tender. Serve immediately.