Preheat your oven to 400 degrees and line a baking sheet with parchment paper.
Place the acorn squash halves open-side down on the baking sheet and roast in the preheated oven for about 20-30 minutes, or until tender.
While the squash roasts, heat the oil in a medium skillet over low heat and add all the onions, stirring to coat. Sprinkle with a bit of salt and cook over low heat, stirring every 5 minutes ago to prevent burning. Once onions have been cooking for about 25 minutes and are deep golden brown, remove from heat and set aside.
Heat the rosemary oil in a large saucepan over medium heat and the sausage stirring to break up lumps until just browned. Add the apples, rosemary rub and thyme and continue to cook, stirring until the apples soften. Add the spinach and cook until the spinach wilts.
Once squash is removed, turn the oven up to broil.
Add the caramelized onions to the sausage mixture and stuff each squad half with the mixture. Arrange the squash on the baking sheet, stuffing side up, and put under the broiler, checking frequently until the tops are golden brown.