Preheat your oven to 400°F (200°C). Slice the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle insides with olive oil and season with salt and pepper.

Place squash cut-side up on a baking sheet and roast for 35–40 minutes until tender.

In a skillet, heat olive oil over medium heat. Add vegetarian sausage, red bell pepper, and onion; cook for 6–8 minutes. Stir in garlic and cook for another 1–2 minutes.

Add balsamic vinegar to deglaze the skillet, then stir in marinara sauce and simmer for about 5 minutes. Mix in shredded squash flesh after roasting.

Stuff squash halves with the filling and top with mozzarella cheese.

Lower oven temperature to 350°F (175°C) and bake for an additional 5–10 minutes until cheese is bubbly. Broil for an extra minute if desired.

Garnish with parsley before serving.