Heat a large skillet over medium-high heat and melt butter.
When it begins to foam, add mushrooms and onions and sauté until they begin to caramelize.
Deglaze the pan with sherry, season with salt and pepper and cook down, scraping up any brown bits from the bottom of the pan.
When the sherry has reduced, add spinach and stir until it has wilted.
Adjust seasoning and serve immediately.
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