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1
Cook the spinach in the water clinging to its leaves, then transfer it to a clean cloth. Twist the cloth firmly and repeatedly until no further moisture emerges. Chop the spinach fine. Separately, pass the ricotta through the fine sieve and leave it to drain for several hours or overnight. The spinach must be dry to the touch and the ricotta must hold its shape when pressed with a spoon.
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2
Combine the spinach, drained ricotta, egg, grated Parmigiano-Reggiano, and nutmeg in a bowl. Season assertively with salt and pepper — the filling must carry more flavour than seems necessary, as the pasta wrapper will dilute it. Taste the mixture directly and adjust. The filling should be cohesive, thick enough to hold its shape on a spoon, and bold on the palate.
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3
Make the sfoglia: mound the flour, form a well, break the eggs into the centre, and work to a smooth, firm dough. Rest it wrapped for 30 minutes, then roll to a sheet slightly thicker than for tortellini — the larger format tolerates this without becoming doughy. Cut into 5–6 cm squares.
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4
This is where the dish lives or dies. Place a generous tablespoon of filling off-centre on each square. Fold the pasta over the filling to form a half-moon or triangle, pressing firmly from the filling outward to expel every pocket of air before sealing the edges. Any air trapped inside will expand during cooking and burst the tortelloni. Run a fingertip moistened with water along the edges before pressing if the dough has dried.
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5
Bring a large pan of generously salted water to the boil, then reduce to a gentle simmer — a rolling boil will tumble the tortelloni violently and rupture them. Lower the tortelloni in batches. Cook for 3–4 minutes. They are nearly done when they float to the surface; taste one to confirm the pasta is just tender.
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6
Make the burro e salvia while the tortelloni cook. Start with the cold butter in a cold pan and set over a medium flame. As the butter melts and begins to foam, add the sage leaves. Continue cooking until the butter reaches the nocciola stage — golden-brown with a hazelnut scent — and the sage leaves are crisp. Remove from the heat immediately; the line between nocciola and burnt is narrow.
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7
Lift the tortelloni from the water with a slotted spoon and transfer them to the pan of sage butter. Add a splash of pasta cooking water and toss very gently — they are fragile. The water and butter will form a light emulsion that coats each tortelloni. Serve immediately with grated Parmigiano-Reggiano.