In a large skillet, cook the pork breakfast sausage over medium heat, breaking it into pieces. Once browned and fully cooked after about 8-10 minutes, add the finely chopped yellow onion. Sauté until translucent, approximately 5 minutes, then remove from heat and let cool.

In a spacious mixing bowl, combine the cooled sausage and onion mixture with cubed stale sourdough bread and shredded Gruyère cheese. Toss gently until evenly coated.

In a separate bowl, whisk together eggs, whole milk, heavy cream, and Dijon mustard until smooth. Season with salt, black pepper, and ground nutmeg.

Pour the custard mixture over the bread, sausage, and cheese blend, gently folding the ingredients together to ensure all bread chunks are soaked.

Cover the bowl with plastic wrap and refrigerate for at least 2 hours or overnight.

Preheat your oven to 350°F (175°C) before removing the casserole from the refrigerator. Uncover and bake for 45-55 minutes until golden brown and puffed.

Let the casserole cool for about 10 minutes before slicing. Serve warm, optionally garnished with fresh herbs.