Prepare the Filling: Wash the spinach thoroughly.

In a large pot, cook the spinach, covered, using only the water clinging to its leaves until completely wilted.

Drain and, once cool enough to handle, squeeze out as much excess moisture as possible.

Chop the spinach finely.

Sauté the Aromatics: In a medium skillet, melt 3 tablespoons (45 ml / 1.5 fl oz) of butter over medium heat.

Sauté the chopped onion and garlic until golden and fragrant.

Stir in the chopped spinach, vinegar, ½ teaspoon salt, oregano, and nutmeg.

Cook for a few more minutes, stirring, until the mixture is thick and well-combined.

Taste and adjust seasoning if needed, then set aside to cool.

Prepare the Dough: Boil the potatoes in their skins until tender.

Peel them while still hot and pass them through a potato ricer or a coarse sieve into a large bowl. Allow the potato to cool slightly.

Mix the Dough: Lightly beat the 2 whole eggs and 1 egg yolk.

Stir the beaten eggs into the riced potatoes along with 1½ teaspoons of salt and the nutmeg. Gradually add 2 cups (480 ml) of flour, mixing with your hand until a smooth dough forms.

Add more flour as needed to create a dough that is stiff enough to hold its shape but is not overly dry.

Roll it Out: Generously flour a large sheet of parchment or waxed paper.

Place the dough on top and roll it into an 11x13 inch rectangle, dusting with flour as needed to prevent sticking.

Assemble the Roulade: Spread the cooled spinach filling evenly over the dough, leaving a 1-inch border on the two short ends and one long side. Leave a 2-inch border on the remaining long side. Starting from the long side with the 1-inch border, tightly roll the dough into a log, using the paper to help lift and guide it. Pinch the seam and the ends securely to seal the filling inside.

Wrap for Cooking: Carefully wrap the entire roulade in a large piece of cheesecloth, ensuring you have 2-3 layers. The cheesecloth should be several inches longer than the roulade on both ends. Tie the ends securely with kitchen twine, like a sausage. Tie another piece of twine around the middle to help it keep its shape.

Poach the Roulade: Gently lower the wrapped roulade into a large roasting pan or stockpot.

Cover it completely with boiling, salted water.

Simmer gently for 50 minutes, carefully turning it over halfway through the cooking time.

Rest and Unwrap: Carefully lift the roulade from the water and let it rest on a cutting board for 5 minutes. Snip the twine and gently unwrap the cheesecloth. If it sticks, use a sharp knife to help peel it away.

Slice and Serve: Using a sharp knife, cut the roulade into ¾-inch thick slices.

Serve immediately on warm plates, generously drizzled with melted butter and sprinkled with freshly grated Parmesan cheese.

Dry potatoes are your best friend.

After boiling and peeling, let the potatoes sit for a few minutes to steam dry. Excess moisture will make the dough sticky, requiring too much flour and resulting in a tough roulade.

Squeeze that spinach! The filling must be as dry as possible. Use your hands, a potato ricer, or a clean kitchen towel to wring out every last drop of water to prevent a soggy interior.

Don't overwork the dough.

Mix just until the are combined. Overworking develops gluten, which will make your gnocchi-like dough chewy and dense instead of light and tender.