To make the sauce, melt the butter in a pan set over a medium heat, add the flour, and whisk well.
Pour over the milk, and continue to whisk until the sauce thickens.
Add the grated cheese, dijon mustard, salt and pepper, and mix well, then remove from the heat.
Divide the sauce, so that 3 quarters of it can go towards the filling, and the rest for assembling the lasagna.
Add the chicken, artichokes and spinach, making sure you squeeze the spinach well, so it's not watery.
Brush the dish with a thin layer of white sauce, arrange one layer of lasagna sheets (see photo no 2), then spread half of the filling over.
Arrange another layer of lasagna sheets, again filling, and top with other sheets.
Spread the remaining white sauce all over the top lasagna sheets, and cover with grated cheese.
Bake in the preheated oven at 180 degrees Celsius (350 Fahreneheit) for about 20 minutes, or until the top is golden and the lasagna is bubbling hot.