Sift the flour onto a clean work surface, creating a mound with a flat top. Use your fingers to make a deep well in the center of the flour, then crack the eggs into this well. Add the salt and oil to the eggs. Using your fingertips, begin gradually incorporating the flour from the inner walls of the well into the egg mixture, working in a circular motion. Continue mixing until a sticky, shaggy ball of dough begins to form.
Transfer the dough to a lightly floured work surface and knead it vigorously for about 10 minutes. Use the heels of your hands to push the dough away from you, then fold it back over itself and rotate it a quarter turn. Repeat this process until the pasta dough becomes perfectly smooth, elastic, and no longer sticks to your work surface or hands.
Shape the kneaded dough into a smooth ball and place it inside a lightly oiled plastic bag to prevent it from drying out. Let it rest at room temperature for approximately 30 minutes—this resting period allows the gluten to relax, making the dough easier to roll. After resting, roll out the dough on a lightly floured work surface, working from the center outward and rotating frequently, until you achieve a very thin 15 in (37 cm) square sheet. Leave this pasta sheet uncovered on the counter for about 20 minutes to dry out slightly, which will help prevent the ravioli from becoming soggy.
While the pasta is resting, prepare the filling: in a mixing bowl, beat together the ricotta cheese, half of the grated Parmesan, the egg, a pinch of nutmeg, and the well-drained spinach. Mix until everything is thoroughly combined and smooth. Season the filling generously with salt and pepper to taste.
Once your pasta sheet is ready, spoon small portions of filling onto one half of the pasta, spacing them evenly. Fold the other half over, press around each mound to seal, and cut into individual ravioli squares. Cook the ravioli in a large pot of boiling salted water for 3–4 minutes until they float to the surface and are tender. Drain carefully using a slotted spoon. Serve the ravioli immediately, tossed with melted butter, a generous sprinkle of the remaining Parmesan cheese, and a good grinding of black pepper over the top.
