Prepare one cup of rice according to the directions on the package, substituting chicken stock for water.
Meanwhile, melt butter (or coconut oil) in a large skillet over medium heat. When the butter begins to sizzle, add the onions and cook for 2 minutes. Add the mushrooms and continue to cook until the onions are nearly translucent. Season with salt & pepper.
Add spinach. Stir often to cook, reducing heat if rice is not done yet.