Stabilize your grill or oven to 350°F. Cut the spaghetti squash in half and use a spoon to scrape out the seed. Place cut side down on a half sheet pan with ½ inch of water. Roast for 45 minutes to 1 hour, until the squash is fork tender.
During the last 15 minutes of cooking, place the sliced sausage in a grill-safe pan and place on the grill to render and cook.
Remove the sausage and roasted squash from the grill and allow to cool slightly.
When the squash is cool enough to handle, use a fork to gently scrape the sides into strands and toss it in with the rendered sausage. Fold in marinara and ½ of the shredded mozzarella. Fully incorporate and transfer the mixture back into the skin of the squash.
Return the stuffed squash back on the sheet pan cut side up, top with the other half of the shredded mozzarella and place dollops of pesto on top.
Return the squash back to the grill and bake for another 20 to 30 minutes, or until the filling is hot and the cheese is melted.
Remove from the grill. Enjoy!