Clean the fresh pioppini, cutting a thin slice off the bottom of each stem, rinse in cool water to remove any dirt, then pat dry with paper towels.
Heat the oil in a large skillet and sauté the chopped onion for a few minutes. Add the pioppini mushrooms, season with salt and pepper, and cook for 10 minutes on low heat.
In the meantime, in a large pan melt the butter over medium-low heat. Add the gnocchi and sauté for 5 minutes, then pour them in the pan with the mushrooms, stirring frequently. Serve the gnocchi on a plate, adding a few leaves of baby spinach and Parmigiano Reggiano shavings.