Have the ragù Bolognese ready.

Make the homemade spinach lasagne sheets.

• Pour the flour on a wooden working surface and shape into a mound with a large well in the centre. Crack in the egg, then add the finely minced spinach. Pour in the olive oil and add a pinch of salt.
  
• Using a fork, stir slowly, starting from the centre and gradually picking up more flour from the edges, whisking as if you are beating eggs for an omelette. When the dough turns crumbly, switch to kneading with your hands.
• Continue kneading the ball of dough until the gluten starts to develop, as this will render the sheets of pasta stronger. When the ball of dough is smooth, silky, and no longer sticky, wrap it in plastic wrap, or cover with a bowl, then let rest for 30 minutes at room temperature before using it.

Prepare the béchamel sauce.

• Melt the butter in a saucepan over medium heat. When melted, spoon in the flour and whisk for a few minutes until golden and toasted. Pour in the cold milk in a thin stream, stirring constantly to avoid lumps.
• Cook the béchamel sauce for a few minutes, still stirring constantly, until thickened. Your whisk should leave visible trails in the sauce.
• Season with a good pinch of salt and grated nutmeg.

Now prepare the lasagne.

• Preheat the oven to 180 degrees C (350 degrees F).
• Roll out the dough into paper-thin sheets of pasta. Leave the pasta sheets on a tablecloth dusted with semolina flour.
  
• Bring a large pot of water to the boil. When it boils, add salt and cook the green lasagne sheets. Cook the sheets one at a time, plunging them for a minute and then immediately passing them under cold water. Place them on a clean cloth to absorb the excess water.
• Have all the other ingredients ready nearby. If the béchamel sauce and the Bolognaise ragù are cold, reheat them until warm. Grease with butter a 20x30cm (11- x 15-inch) baking dish.
• Spread some of the meat sauce across the bottom of the baking dish, just enough to make a thin, even layer. Then line it with enough sheets of pasta to cover it. Spoon some of the ragù Bolognese over the pasta sheets and spread it into a thin, even layer. Then spoon some béchamel sauce over the ragù and spread it. Sprinkle with grated Parmigiano Reggiano. Cover the béchamel sauce with more sheets of pasta, and repeat for four more layers.
  
• Distribute a few slivers of butter over the top.
• Bake for about 45 minutes, or until golden brown and bubbling.
• Serve immediately, or better yet, let it rest for a few hours, or even better, until the next day, and reheat just before serving.