In a large skillet or pot over medium-high heat, sauté the onion, garlic, and mushrooms in olive oil for 3–4 minutes until softened and fragrant.
Sprinkle the flour over the vegetables and stir constantly for 1 minute to cook out the raw flour taste.
Reduce the heat to medium-low.
Add the fresh spinach (it will be a large volume but will cook down) and the soy milk.
Cook, stirring occasionally, for 8–10 minutes until the spinach is fully wilted and the sauce has thickened.
Remove from heat and stir in the vegan margarine and nutmeg until the margarine is melted.
Season generously with salt and pepper to your liking before serving.
Don't overcrowd the pan when cooking the mushrooms.
Sauté them in a single layer to ensure they brown properly rather than steam.