Prepare cream sauce and set aside, keeping warm: Melt 3 Tbsp butter in a medium sized sauce pan over medium heat. Slowly add 3 Tbsp cornstarch and stir for about 1 minute. While whisking constantly, slowly add 2 c. whipping cream (or half and half) and 1 c milk until well combined. Stir in 3 Tbsp grated Parmesan cheese. Reduce heat to low and whisk occasionally for about 10 minutes until sauce becomes rich and creamy, adding more milk, if needed. You want it to be the consistency of a thin gravy. Season with salt and pepper, to taste. Preheat oven to 400-degrees. Spray a 9x13 baking dish with cooking spray and place (4)-6 cod fillets in the dish. Sprinkle with salt. Combine 4 Tbsp. butter melted and 2 cloves garlic minced and brush over fillets, reserving any leftover butter mixture. Sprinkle 1/2 c seasoned breadcrumbs over fillets and pour remaining butter mixture over the top. Pour cream sauce in the spaces between the cod fillets so it fills up the baking dish and just the tops of the fillets are visible. Place in the preheated oven and bake for 25-30 minutes or until the cream sauce starts bubbling. When the sauce starts bubbling up, set oven to "broil" and broil for about 3-5 minutes or until the fillets start to turn golden brown. Keep a close eye on it to make sure it doesn't start to burn. Remove from oven and serve with potatoes, rice, or vegetables.