Add the ground spicy Italian sausage, crushed tomatoes, chopped onion, minced garlic, Italian seasoning, red pepper flakes, salt, and black pepper to the slow cooker. Stir to combine and break up the sausage as much as possible.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours, until the sausage is fully cooked and the sauce has thickened slightly.
About 45 minutes before the sauce is done, preheat the oven to 400°F. Cut the spaghetti squash in half lengthwise and scoop out the seeds. Place the halves cut-side down on a baking sheet and bake for 30 to 40 minutes, until the flesh is tender.
Let the squash cool for a few minutes, then use a fork to scrape the flesh into strands. Divide the strands among four plates.
Spoon the sausage ragu over the squash strands. Top each serving with chopped parsley and grated Parmesan cheese.