Cook spinach leaves in a pan with 2 tablespoons of water for 5 to 6 minutes. Season with salt and pepper; drain well and squeeze them in a clean drying cloth to remove excess water. Finely chop the leaves; set aside to cool.
Place flour in a mound on a wooden board or clean working surface. Make a hole in the centre and add eggs, salt and cooled spinach. Gently mix everything together to form a dough; knead for at least 8-10 minutes, until dough is smooth. Wrap the pasta dough in cling film and rest in a cool place (but not in the fridge) for one hour.
Transfer drained ricotta to a bowl; add yolk, grated Parmesan cheese, salt, pepper and nutmeg. Mix with a fork to create a soft mixture.
Divide the spinach pasta dough into 6 equal pieces. Take one piece at a time (covering the remaining pasta with cling film) and roll into a thin sheet 10cm long and 5cm wide with a rolling pin or pasta machine.
Place 1/2 tablespoon of filling about 1/2cm from the edge of the long side of the dough; continue placing 3 more portions of filling, 1cm apart, until you reach other side. Fold dough over to cover filling and press down on edges to seal. Slice into 4 ravioli and press down with your thumb to seal all the edges. Repeat with remaining spinach pasta strips.
Arrange ravioli on a clean drying cloth, separated from each other so they don't stick. If you are not planning to cook them immediately, leave them to dry for 1 hour, turning them from time to time so that they dry equally on both sides. Cover them with cling film and store in the fridge.
Bring a large pot of lightly salted water to the boil; add 1 tablespoon of oil and ravioli. Cook for 2 to 3 minutes or until the ravioli rise to the top. Scoop ravioli out with a slotted spoon and serve with a warm tomato sauce.