Preheat oven to 350 degrees F. Lightly spray a 1-1.5 quart baking dish. A 9-inch square would probably be good too.
Brown sausage and onion in a large skillet over medium heat. Drain sausage well. Set aside in a large bowl. Add the shredded potatoes to this bowl as well
In a large saucepan, melt the butter over medium. Whisk in the flour until you have a smooth paste. Cook until this bubbles and cook for about 1 minute.
Gradually add the milk and chicken stock, whisking constantly so no lumps form. Cook until sauce begins to thicken and starts to bubble around the edge of the pan.
Add the Velveeta and reduce heat to low. Stir constantly until the Velveeta is melted.
Pour the sauce over the sausage/potato combination in the bowl—the amount of sauce you use it up to you. I used it all because I wanted it creamy. The consistency will be the same cooked as before, so let your eye—and your taste—be your guide. Mix well and turn into prepared pan.
Sprinkle top with Panko crumbs if using.
Bake at 350 degrees F for 45-60 minutes.
Recipe may be doubled to serve 4.