Remove the fennel from the stalk and cut into strips lengthways with the greens. Fry the onion and fennel in the oil for about 5 minutes.
Melt 1 tablespoon butter in the saucepan, add the flour while stirring constantly and let it swell over a low heat. Gradually pour in the warm milk while stirring. Bring to the boil, simmer for a few minutes so that the flour taste disappears and season with lemon juice, freshly grated nutmeg, salt and pepper!
Grease a baking dish a little, fill in the pasta, pour the fennel over it and pour the sauce over everything. Mix lightly and sprinkle the parmesan on top.
In the oven at 190 degrees (convection 180 degrees) and bake on the middle rack for 20 minutes.