Roughly chop the spinach. Peel the onions and garlic and cut into small cubes. Sweat in the butter, add the spinach, cook briefly, season with salt and pepper. Put the spinach in a bowl, mix in the tuna and let cool down well.
For the sauce, melt the butter in a saucepan, stir in the flour and add hot milk while stirring. Let it cook well, then add the parmesan and cream. Season the sauce with salt and nutmeg.
Preheat the oven to 200 ° C. Grease a baking dish with butter. Put the filling in a dressing sack and press it into the dough rolls. Place the cannelloni in the pan, pour the sauce over it and bake in the oven for about 35 minutes.