Pesto Spaghetti Squash

1 spaghetti squash, halved lengthwise and seeded
3 tablespoons butter, divided
1 onion, thinly sliced
2 cloves garlic, grated
1/2-1 lb Italian sausage
2 cups spinach, lightly chopped
1 teaspoon garlic salt
1 teaspoon Italian seasoning
1 teaspoon olive oil
2 tablespoons prepared pesto
1/2 cup grated Parmesan cheese

Preheat oven to 400 degrees F. Grease a baking sheet. Place squash, cut sides down, on prepared baking sheet. Bake until cooked through about 1 hour. Remove from oven; cool for 10 minutes. Once squash is cool enough to handle, scrape flesh into sting-like strands with a fork. Place in a bowl and set aside.
Melt 1 tablespoon butter in a large skillet over medium-high heat. Add onions,garlic and sausage. Cook until onion turns translucent and meat is browned about 10 min. Stir in spinach and reduce heat to med-low. Stir in squash, remaining 2 tablespoons butter, garlic salt,Italian seasoning; cook for 2 minutes. Remove from stove and place squash mixture in large bowl. Stir in olive oil and pesto into the squash mixture. Slowly add grated Parmesan cheese, stirring until evenly mixed.