Roast the squash. Preheat the oven to 375°F. Line a sheet tray with parchment paper. Cut a small piece of the tops and tips off the acorn squash so they will lay flat on the sheet tray. Cut each acorn squash horizontally across the center and scoop out any stringy, seeded pieces.
Poke each piece of squash several times with a fork, drizzle with 3 tablespoons of olive oil and season with salt and pepper. Roast in a 375°F oven for 35 minutes. While the squash roasts, prepare the stuffing.
Sauté the stuffing. In a large sauté pan, heat the remaining 3 tablespoons of olive oil over medium-high to high heat. Once the oil shimmers, add the sweet potatoes and onions, cook for about 5 minutes stirring frequently until the sweet potatoes are just starting to soften and the onions are starting to brown. Combine the apple with the ingredients in the pan, season with salt and pepper, and sauté for 1 minute longer.
Using a spatula or wooden spoon, break apart the sausage into bite sized pieces as it cooks. Once the sausage is almost completely browned, about 3 to 4 minutes, deglaze the plan with the marsala wine. Allow the wine to reduce, about 1 minute, then add the chicken stock to the pan. Reduce the heat to medium-low and simmer until almost all the liquid is absorbed. Remove from heat until the acorn squash is done roasting.
Stuff and roast. Remove the roasted squash from the oven. Scoop the filling and any pan juices into each of the acorn squash halved, pressing down gently to fill. Return the squash to the oven and roast for 10 minutes. Serve immediately.