Melt butter in medium saucepan over medium heat. Cook eschalots and mushrooms 5 min until mushrooms are soft. Stir in flour for 30 sec. Add milk and cream, stir until incorporated, then stir in salt, pepper and nutmeg. It will heat up as it thickens. When you see a bubble, cook 30 seconds then take off the stove.
Sprinkle scallops with salt, divide between small shallow gratin dishes in single layer (11cm / 4.3" diameter, 2.5cm / 1" deep is ideal, you can also use shells). Cover with all the bechamel, top with cheesy crust.
Put on tray, bake 20 minutes at 200°C / 400°F (180°C fan) until top is golden. Rest 2 minutes then serve!
