Preheat the oven to 350 degrees then spray a 9x13" baking dish with nonstick spray and set it aside.
Heat a large skillet over medium-high heat then add the breakfast sausage and shallot or onion and cook through. Drain if necessary then return the sausage to the pan and set aside to cool slightly (can be done ahead of time).
To a very large mixing bowl, add the eggs, salt, and pepper then whisk to combine. Add the sour cream and milk then whisk to combine. Add 3/4 of the shredded cheese, the cooked and slightly cooled breakfast sausage, and thawed hash browns then stir to combine.
Pour the mixture into the prepared baking dish then spread into an even layer. Sprinkle the remaining cheese on top then cover the dish with a piece of nonstick-sprayed foil and bake for 60-75 minutes or until the center has set and a knife inserted into the center comes out clean. Remove the foil then bake for 5 more minutes or until the cheese has turned golden brown.
Let the casserole sit for 15-20 minutes before slicing and serving.
Video
Notes
If the mixture seems a little dry before pouring into the casserole dish, add another one or two eggs whisked with milk.