If the béchamel is too thick thin with chicken broth or white wine.
Any veggies you have on hand can be added to the mushroom layer: zucchini, leaks, or celery.
If you don’t have fresh spinach you can use 2 boxes of fresh, or if you don’t have frozen add 2-4 cups more of fresh. I like the mix of the two but have been known to just use one or the other.