Place the flour, salt, two teaspoons of olive oil and egg in a bowl and knead into a stiff dough. Cover and set aside for ten minutes. Divide the dough into four portions. Work with one portion of dough and keep the rest of the portions covered with a damp cloth.
Flatten the dough and roll out into a rectangle. Fold the sheet thrice and roll out again. Repeat this process ten times till the sheets can be rolled very thin. Boil six cups of water with salt; add the pasta sheets.
When the sheets rise to the surface remove them with a slotted spoon and refresh in cold water. Drain and lay sheets on a kitchen towel. Brush each sheet with the remaining olive oil and place one on top of the other. For the filling, melt half the butter in a pan; remove from heat and stir in five teaspoons of flour.
Sauté over low heat for two minutes. Gradually add the hot milk and whisk till smooth. Simmer for ten minutes, stirring continuously, till the sauce coats the back of a spoon. Remove from heat, strain and set aside.
Preheat the oven to 190°C. In a separate pan, melt the remaining butter over low heat; cook onion and garlic till soft, making sure that the colour does not change. Add the spinach and mushrooms and cook over medium heat for two to three minutes.
Season with salt, white pepper powder and nutmeg. Add half the Parmesan cheese and enough white sauce to bind it all together. Lay the pasta sheets on a board. Spoon the filling along the centre of each sheet and roll up the pasta.
Place the rolls in a buttered ovenproof dish and pour the remaining sauce over. Sprinkle the remaining Parmesan cheese mixed with breadcrumbs over the top.
Bake for twenty to twenty-five minutes till the top is golden brown.