Empty the frozen spinach into a big saucepan with about 1/2 cup of water. Bring to high heat and steam until the spinach is defrosted. Drain in a collander and squeeze out as much liquid as possible. Set aside.
Wipe out the same saucepan and melt the margarine. Add the flour and whisk it for a couple of minutes until it's all bubbly and smoooth.
Slowly add in the stock, whisking all the while to make a smooth sauce.
Add in the sliced mushrooms and let them steam until tender (keep whisking every once in a while so you don't get any lumps!)
Whisk in the soymilk.
Add the spinach and stir to coat with the sauce.
Add the nutmeg and season to taste with pepper and more salt if desired.