Preheat the oven to 425º Fahrenheit.
Begin by using a very sharp knife to cut off the top stem of the squash. Then, very carefully cut the squash in half longways.
Use a metal spoon to scrape the seeds out of the inside until completely clean.
Spray or drizzle the inside of the squash with olive oil, along with the sheet pan.
Roast at 425º Fahrenheit for 20 minutes, until the squash is fork tender.
Remove and set to the side.
Crumble the meatless sausage with your fingers, or mince with a knife.
Finely chop the mushroom caps and give the arugula a rough chop.
Remove rosemary from the stems and roughly chop it into small pieces.
In a nonstick pan, add about 1 tablespoon olive oil and the crumbled sausage.
Sauté until browned.
Add the chopped mushrooms and continue to sauté until cooked.
Remove the pan from the heat and add rosemary and arugula and grated parmesan.
Stir to combine until the arugula has wilted and the parmesan is mostly melted.
Turn the oven to 300º Fahrenheit.
Gently scoop the sausage filling into the cavity of the squash.
Fill until just slightly over the top of the squash.
Top with more grated cheese and put back into the oven for 5 minutes or until the parmesan starts to melt and/or brown.
Optional, but you may want to broil for 2-3 minutes to brown the cheese further.
Serve warm and enjoy!