Preheat oven to 425ºF.
Preheat a medium, nonstick skillet over medium-high heat with 2 tablespoons EVOO. Add the mushrooms and sauté until golden brown. Season them with salt and pepper then remove them to a plate.
Lower the heat to medium-low and in the same skillet the mushrooms were cooked in, add the butter to the pan and melt. Add the garlic and stir 1 minute, sprinkle the flour into the pan and whisk in the milk and season with salt, pepper, and nutmeg.
Add the mushrooms back to the skillet, turn to combine with sauce. Separate the drained defrosted spinach with your fingers to loosen it up as you add it to the pan, stir the spinach into mushrooms and sauce. Remove from heat.