Add the feta cheese and spinach to the food processor and blend well until creamy. Add 1-2 tablespoons of water if needed to help it process.
Start stuffing the cannelloni using a piping bag or a teaspoon. Stuff them until they are full.
Spray an oven tray with some olive oil.
Place the stuffed cannelloni in the oven tray in a single layer.
In another bowl, combine the tomato paste, peeled and chopped tomatoes, basil, oregano, salt, and pepper. Mix well, then pour over the cannelloni in the tray and spread evenly.
Add some water to the tray; not much, just a little, about 1/4 cup.
Cover with aluminum foil and place in the oven at 180C (392F).
Bake for about half an hour, until the cannelloni are tender and the sauce is bubbling, then serve.