Dice and sauté the onion. Add the spinach and bring to the boil briefly. Pour through a sieve. Pour the cream into the collected liquid and bring to the boil. Crumble the feta into the spinach. Fill the canneloni with the mixture. Put in a greased casserole dish, pour the liquid over it (if necessary stretch with vegetable stock). Sprinkle the grated cheese on top.
Bake at approx. 200 ° for 25-30 minutes (the canneloni usually take longer than what is written on the package). Also tastes good as a lasagne when there are no canneloni.