Slice squash open vertically. Place on a sheet tray and rub 1 tablespoon olive oil over the cut surface of the squash. Sprinkle with half a teaspoon of salt. Roast for 40-50 minutes until soft. Remove from oven and let cool for 10 minutes. Using a spoon, remove the seeds. Then, using a fork, pull out the soft flesh of the squash and place in a bowl to rest. Set aside squash boats on a sheet tray.
Heat 1 tablespoon olive oil in a large pan. Add onions and 1 teaspoon of salt and cook over low heat stirring occasionally until browned and softened, about 15 minutes.
Add sausage and break up with spatula. Cook, stirring occasionally, until mostly cooked. Add butter and squeeze in juice of half a lemon.
Add in spaghetti squash flesh, spinach, 1 teaspoon salt and pepper. Saute for 5 minutes. Remove from heat and stir in the juice from the other half of the lemon.
Fill squash boats with filling. Lower oven temp to 375º and bake boats for 15 minutes.
Dry toast breadcrumbs in a small skillet for 1-2 minutes. Remove and put in a bowl. In the same skillet, heat 1 teaspoon olive oil and saute garlic until fragrant, 1 minute. Add to breadcrumbs. Mix in parsley and parmesan. Top boats with breadcrumbs mixture.