I know it sounds odd, but my girl LOVED butternut squash! This dog might even go for it before a steak, especially if there were also pasta and cheese involved in the deal!
Ensure your onion is grated very finely to help it dissolve smoothly into the sauce base. When making the roux, cook the flour for at least 1-2 minutes to eliminate the raw flour taste. Add the stock gradually, whisking continuously to prevent lumps. Remove the pot from direct heat or lower the heat significantly before stirring in the cheeses in a figure-eight motion; this helps prevent the cheese from seizing or becoming grainy. Taste and adjust seasoning with salt, nutmeg, and pepper at the end.