ZUPPA DI PATATE CIPOLLE

Ingredients

• 3 tablespoons butter
• 3 tablespoons extra-virgin olive oil
• 1½ pounds (750 g) yellow onions, peeled and very thinly sliced
• Salt to taste
• 3 ½ cups homemade beef stock
• 2 pounds (900 g) boiling potatoes, peeled and diced into ¼ inch (6 mm) cubes
• 3 tablespoons freshly grated Parmesan cheese