Preheat oven to 400 degrees F. Coat a large baking sheet with cooking spray.
Remove the outer skin from onions, keeping them whole. Cut onions from the top almost to the bottom (without cutting all the way through), making 8 equal wedges. Place onions on baking sheet, spray with cooking spray and season with salt and pepper. Arrange spaghetti squash on the same baking sheet, flesh side up. Spray squash with cooking spray and season with salt and pepper. Roast squash and onions for 25 to 30 minutes, until both are tender and onions are golden brown.
Meanwhile, heat oil in a large skillet over medium heat. Season both sides of pork with salt and pepper. Add pork to hot skillet and cook 2 minutes per side, until golden brown. Remove pork from pan and set aside. To the same skillet add mushrooms and garlic and saute 5 minutes, until mushrooms soften and release juice. Add chicken broth and bring to a simmer. Return pork to pan and simmer 5 minutes, until cooked through. Add heavy cream and simmer 1 minute. Using a fork, pull squash from the skin in shreds, making spaghetti-like strands. Arrange squash on a platter or plate and top with pork and cream sauce. Top with hazelnuts and parsley. Serve 2 roasted onions alongside, with ranch dip, reserving remaining onions for another meal.