In a pot, melt the butter, then brown the chopped onion. When the onion begins turning slightly golden, add the spinach.
Once the water from the spinach has totally evaporated, remove the pot from the heat and add half of the Fourme d’Ambert cheese, which should be cut into small pieces.
Season with table salt and freshly ground pepper, and keep the mixture at room temperature.
At the same time, prepare the béchamel sauce. To do this, in a pot, melt the butter, then sprinkle in the flour. Allow the flour to cook gently without browning. Once the flour and butter mixture is frothy, add the milk, then whisk the mixture briskly and let it cook until you have a smooth béchamel without lumps. Season with table salt and freshly ground pepper.
Add half of the béchamel sauce to the spinach and fill the cannelloni tubes one by one with the mixture.
Place the stuffed cannelloni tightly together in an oven-safe dish. Pour the rest of the béchamel over the cannelloni and scatter little triangles of the remaining Fourme d’Ambert cheese over the top.
Bake in the oven for 40 minutes.
Remove the dish from the oven, scatter the crushed hazelnuts over the top and serve nice and hot.