Heat the olive oil in a pan and sauté the onions and garlic. Add the spinach, pour in the water and steam for approx. 2 minutes.
Crumble the feta into small pieces and fold into the spinach. Season to taste with salt, pepper and nutmeg.
Fill the cannelloni with the mixture and place in a baking dish.
Mix the tomatoes with the balsamic vinegar and oregano and season with salt and pepper. Pour over the cannelloni and sprinkle with cheese (amount to taste - I always use a lot).
Bake in the preheated oven on the middle rack at 200 ° C for approx. 30 minutes.