Heat your oven to 425ºF and lightly grease an 8- or 9-inch baking dish with non-stick cooking spray.
Place a large skillet over medium heat. Add in the mushrooms and season with salt and pepper. Cook the mushrooms in this dry skillet until they release some of their moisture and begin to pick up a little colour, about 5 minutes.
Add in the butter and shallot and continue to cook for 3 minutes or until the shallot is tender and lightly golden. Stir in the garlic and cook for 30 seconds. Sprinkle over the flour, stir well to combine, and allow the flour to toast for about a minute. Switch to a whisk and, while whisking constantly, slowly incorporate the vegetable broth and milk. Bring to a simmer and cook until thickened, about 1 to 2 minutes.
Whisk in the mustard, cheese, and nutmeg and season with more salt and pepper, if needed. Stir in the spinach and parsley and transfer the mixture to the prepared baking dish. Top with the frozen potato tots.
Bake the casserole for 25 to 30 minutes or until golden brown and bubbling. Allow the casserole to rest for 5 minutes before serving.
