Trim the leeks, discard the outer leaves, the extremity of the greens and leathery base with the roots and cut them into slices which are not too thin (about ½ cm).
Transfer them into a capacious colander and rinse them thoroughly under a stream of running water, so as to eliminate any possible residues. Drain well and set aside.
In a large pan with high sides, melt 30 g butter and add the leeks. Salt and pepper lightly and cook for about 7 minutes over medium heat, stirring frequently and adding a little water if necessary to prevent their colouring. The leeks should be slightly wilted and, if possible, not browned.
In a small saucepan, melt the remaining butter and add the rice flour. Stir until the mixture is creamy and cook for a few minutes. Add the rice milk and cream cuisine and season with salt, black pepper and freshly grated nutmeg.
Stir constantly and let the mixture simmer for about 5 minutes, until the sauce has thickened.
Lightly grease a baking dish with a bit of butter and spread the leeks in the middle. Cover with half the sauce and just under half of the Parmesan cheese.
On the other half of the leeks, spread the remaining sauce, a light dusting of paprika and the remaining Parmesan cheese.
Bake in preheated oven at 200 °C for about 10 minutes and make au gratin under the broiler for about 3 minutes or until it has formed a nice, crispy crust on the surface.
Let it cool and serve the gratin just warm or at room temperature.