Preheat the oven to 450 degrees F. Line a baking sheet with parchment.
Heat the oil in a large skillet over medium-high heat. Add the mushrooms and cook, stirring occasionally, until the mushrooms are a rich golden brown, 10 to 12 minutes. Season with salt and pepper. Transfer the mushrooms to a bowl and cool slightly.
Add the spinach, cheese and egg to the mushrooms and stir together to combine (it's nice if there are pockets of cheese in the mixture).
Using a 1/4 cup dry measuring cup as a mold, tightly pack some of the mixture into the cup to make a cake. Tap the cake out of the cup on to the prepared pan. Repeat with the remaining mixture to make 8 cakes. Bake until the cheese is golden, about 12 minutes. Serve hot with whole-grain mustard.
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