This creamy gnocchi dish, topped with cheese and optional truffle slices, is rich and comforting. If truffles aren't available, truffle oil or omitting them still makes for a delicious meal!
Bring a large pot of salted water to a boil. Cook the gnocchi until they float to the top (about 2-3 minutes). Drain and set aside. Preheat the oven to 190°C (380°F).
Heat a skillet over medium heat. Melt 20 g (1½ tosp) butter, then add the garlic and mushrooms. Sauté until golden brown, about 5-7 minutes. Season lightly with salt and pepper.
Make the sauce: In a saucepan, melt 30 g (2 tosp) butter over medium heat. Stir in 2 tbsp flour and cook, stirring constantly with a wooden spoon or spatula, for 1-2 minutes until it bubbles lightly (avoid lumps). Add 60 ml (¼ cup) white wine and cook for 2-3 minutes, stirring. Gradually add 200 ml (¾ cup + 4 tsp) milk in 3-4 parts, stirring to combine. Add the cooked gnocchi and mushrooms, stirring to coat evenly.
Transfer the mixture to a gratin dish. Sprinkle 50 g (½ cup) grated Gruyère cheese on top. Bake at 190°C (380°F) for 20 minutes, or until golden brown and bubbly. Garnish with truffle slices (if using) and chopped fresh chives or parsley. Serve hot.