How to
Peal and chop up the garlic and the onion, fry them in the olive oil together with the spinach. When the oil is gone and they are soft you are done.
Mix in the ricotta cheese, add the oregano, salt and pepper and mix together thoroughly (I might mix with a blender next time to get the spinach in smaller pieces).
Preheat the oven to 435 F.
Pour a layer of the pasta-sauce in the bottom off a oven-safe form, use less than half of the sauce.
Take your lasagne sheets, one by one you put some filling on them and roll them up. Put them with the joint down in the form. When all are done, pour the last of the sauce over and sprinkle the cheese on top.
Put in the oven for about 20 minutes. Serve with a sallad or whatever makes you happy.