Heat a large skillet over medium-high heat. Melt the butter and add in the garlic and cook until fragrant (about one minute). Add in the scallops and cook for 2-3 minutes on each side. Transfer to a plate and set aside. Cook the onion in the butter remaining in the skillet. Pour in the broth, and allow to reduce to half, while scraping any bits off of the bottom of the pan. Next, add sundried tomatoes and cook for 2 minutes. Reduce heat to medium heat, add the milk, Italian seasoning, and parsley, and bring to a gentle simmer, while stirring occasionally. Add cornstarch mixture to center of the pan, and stir, then add in the cheeses. Allow sauce to simmer for a minute until cheese melts through the sauce. If using spinach add now and stir. Continue to simmer while stirring the mixture until the sauce thickens. Add scallops back into the pan and cover in sauce. Serve over cooked pasta or spaghetti squash.