Place the flour and salt on a work surface, make a well and add the eggs, oil and water.
Knead everything to a smooth dough.
Shape into a ball and wrap in a damp tea towel. Leave the dough to rest for 30 minutes.
For the filling, thaw the spinach, place in a cloth and squeeze out the liquid. Chop finely.
Push the ricotta through a sieve, mix with the spinach, Parmesan and egg yolk. Season with salt and pepper.
Thinly roll out the dough on a floured surface and cut out 6cm circles.
Place a hazelnut-sized portion of filling in the centre of each circle.
Brush the dough edges with beaten egg whites, fold over the filling to form crescents and press firmly together.
Twist the tips of the pastries and shape round to a classic tortellini shape. Place the tips on top and press together.
Bring plenty of salted water to the boil in a pan and cook the tortellini in batches for 4-5 minutes.
They are ready as soon as they float to the surface.
Remove the tortellini with a slotted spoon and drain well. Melt the butter and stir in the tortellini. Serve garnished with basil and Parmesan.