First, prepare the squash. Pre-heat the oven to 400 and line a sheet tray with a silicone mat or aluminum foil. Cut each of the spaghetti squash in half lengthwise to have 4 little boats. Scrape the seeds out of each half. Then drizzle the flesh in each half generously with olive oil, followed by generous pinches of the salt, pepper, garlic powder and dried parsley. roast the squash for 45 minutes to make it tender and shreddable.
While it cooks, prepare the filling. Heat the olive oil in a large, deep skillet over medium high heat. Add the mushrooms and sun-dried tomatoes and let them become fragrant for 2-3 minutes. Then add the chicken and let it completely cook while you stir it well. While the chicken cooks, season the entire mixture with the salt, pepper, garlic powder and dried parsley. Once the chicken is cooked through, add the pesto sauce and let it all gently simmer together on low for 5 minutes. Take it off of the heat and set it aside.
When the squash is done, take it out and reduce the oven temperature to 350. Use a fork to completely shred the flesh of the squash into tender "spaghetti", leaving it in its shell. Divide the chicken mixture evenly among the squash boats and stuff them well. Finally, top each stuffed squash with a tablespoon of the parmesan cheese. Put it back into the oven to meld together for 6-8 minutes. Serve immediately and enjoy!