Toast the bread slices either in a toaster or in the hot oven for a couple of minutes. Set aside.
Warm up milk in the microwave for 45 seconds. Set aside.
Melt butter in a saucepan over medium heat and add the flour. Stir it constantly until the flour gains a nice golden colour. Slowly add the warm milk, stirring with a wooden spoon until all milk is incorporated. Continue to stir (switch to a whisk if you feel that the sauce might have lumps) over medium-low heat until the sauce thickens and the spoon leaves a trace on the bottom of the pan. Remove from heat, season with salt, pepper and a pinch of nutmeg. Add 1/3 of the cheese and stir until the cheese melts into the sauce. Set the sauce aside.
Spread butter on one half of the bread and mustard on the other. Place sliced pears on top of the slice with mustard and season with sat. Cover it with some more cheese. Close the sandwich with the buttered slice and place it in the pan with the sauce. Move the sandwich around and turn it upside down a well, to cover it all with the sauce, being careful that the sandwich doesn't open or fall apart.
Transfer sandwich to a baking tray (line it with baking paper for easy clean-up) and bake it in the preheated oven for about 6-7 minutes. Keep an eye on the sandwich to make sure it's not burning too fast. If the top isn't golden brown by the end of that time, turn on the grill for about 2-3 minutes. The sandwich should be bubbly and the cheese melted with some brown spots.
Remove from the oven, transfer to a plate and let it rest for 5 minutes before you serve.