Step 1: Preheat your oven to 400°F (200°C). Carefully cut the spaghetti squash in half lengthwise. Scoop out the seeds and stringy bits. Drizzle the cut sides with 1/2 tablespoon of olive oil, season with a pinch of salt and pepper.
Step 2: Place the squash cut-side down on a baking sheet. Roast for 35-45 minutes, or until the flesh is tender when pierced with a fork. Remove from oven and let cool slightly. Once cool enough to handle, use a fork to scrape out the spaghetti-like strands into a large bowl.
Step 3: While the squash is roasting, heat the remaining 1/2 tablespoon of olive oil in a large oven-safe skillet (or Dutch oven) over medium-high heat. Add the sliced kielbasa and cook until browned on both sides, about 5-7 minutes. Remove the kielbasa and set aside, leaving any drippings in the skillet.
Step 4: Add the chopped onion and bell pepper to the skillet. Sauté for 5-7 minutes, until softened. Add the minced garlic and cook for another minute until fragrant.
Step 5: Stir in the diced tomatoes (with their liquid), chicken or vegetable broth, Italian seasoning, salt, and pepper. Bring to a simmer, then reduce heat and let it cook for 5 minutes to allow flavors to meld.
Step 6: Return the cooked kielbasa to the skillet. Add the spaghetti squash strands to the mixture and gently stir to combine everything evenly.
Step 7: If using, sprinkle the shredded mozzarella cheese over the top of the bake. Transfer the skillet to the preheated oven (if your skillet is oven-safe) or transfer the mixture to a 9x13 inch baking dish.
Step 8: Bake for 15-20 minutes, or until the cheese is melted and bubbly, and the bake is heated through. If desired, you can broil for the last 1-2 minutes for a golden-brown top. Garnish with fresh parsley, if using, and serve hot.