Halve the squash lengthwise and scoop out the seeds. Put the squash, cut-side up, in a large microwave-safe bowl and drizzle with 1 tablespoon olive oil and 1 tablespoon water. Season with salt and pepper. Cover tightly with plastic wrap and microwave until tender, about 20 minutes. Scoop out the flesh into a colander (use a towel to hold the squash) and shred with 2 forks.
Meanwhile, heat a grill pan over medium heat. Grill the sausages, turning, until cooked through, about 20 minutes.
Heat the remaining 2 tablespoons olive oil in a skillet over medium-high heat. Add the bell pepper, onion and 1 teaspoon salt and cook until softened, about 5 minutes. Add the garlic and cook until the vegetables begin to brown, about 4 more minutes. Toss in the squash and parsley and season with salt and pepper. Serve with the sausages and sprinkle with parmesan.