1. Preheat the oven to 450 degrees F. Heat a large skillet over medium-high heat. Add the olive oil and then the mushrooms. 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.
2. 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.
3. Line a baking sheet with parchment. 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.