In a large skillet over medium heat, melt butter until it begins to brown. Add mushrooms, shallots, and garlic. Sauté for about 8 minutes, or until golden brown. Add the wine. Reduce slightly, then add chicken stock/broth, nutmeg, and sage. Simmer for about 8 minutes. Season with salt and pepper to taste.
Add cooked gnocchi to skillet. Add parmesan, reserving about 3 tbsp for serving. Heat for about 1 minute. Add arugula and stir until wilted.
Divide equally among plates. Drizzle with truffle oil and sprinkle leftover parmesan just before serving. Bon appétit!