Add the flour and mix vigorously with a spatula. Cook over low heat for a few minutes, without colouring.
Over medium heat, gradually pour in the cold milk while stirring regularly with a whisk. Leave to cook for 5 to 10 minutes until a smooth bechamel is obtained. Remove from the heat.
Season the béchamel with some salt, pepper and nutmeg.
Add the grated cheese and the egg yolk.
For the spinach
Chisel the shallot. Rinse and drain the spinach.
Heat a knob of butter in a hot frying pan, add the shallot and stir fry for 5 minutes.
Add the spinach to the pan and let it cook for about 5 to 7 minutes, until the leaves have fallen off (softened).
For the eggs
Pour some water in a saucepan and slowly bring it to a boil.
Gently place your eggs in the boiling water with a spoon.
Leave to cook for 6 minutes.
Transfer them to cold water to stop the cooking.
For the presentation
Cut your English muffins in half and toast them.
Peel the soft-boiled eggs and gently cut them in half.
On each slice, place a little spinach fondue, ½ soft-boiled egg, a nice spoonful of Ossetra caviar then coat with Mornay sauce.