Ricotta and Spinach Ravioli (Ravioli di Ricotta e Spinaci)

Ingredients

• 2 cups all-purpose flour, plus more for dusting
• 3 large eggs
• 1 tablespoon olive oil
• A pinch of salt
• 1 cup ricotta cheese
• 1 cup cooked, drained, and finely chopped spinach
• 1/2 cup grated Parmesan cheese
• 1 egg yolk
• Salt
• pepper
• Nutmeg, a pinch (optional)
• 4 tablespoons unsalted butter
• A few sage leaves
• Salt to taste
• Grated Parmesan cheese, for serving