There are four distinct steps to the preparation of this dish. First the spinach, second the eggs, third the Mornay sauce and fourth the final assembly and broiling. None of the steps take a great deal of time and can be done in order and if you prepare everything in advance it should be a smooth operation.
If you have small au gratens do it in individual portions, or a larger au graten or baking dish will also work just fine .
You can use fresh if you like, but frozen works just as well. Defrost the spinach and press out most of the water in a towel.
In a sauté pan melt the butter Once the butter is beginning to brown add the shallot and cook until just wilted. Now add the spinach and nutmeg and salt and pepper and stir to combine and cook for one or two minutes.
If making individual dishes divide the spinach into the four dishes, and set aside, or if using a larger dish spoon the spinach evenly over the bottom of the dish.
Use a pan that can accommodate all the eggs in a single layer and is deep enough to cover them with water.
Using a push pin, puncture the large round end of the eggs, making a small hole (this will prevent them from cracking while cooking). Have a bowl with ice water ready to accept the eggs once cooked.
Bring the water up to a rolling boil. Carefully add the eggs and cook for 6 minutes NO MORE!. Remove the eggs once cooked from the boiling water and place into the ice water to stop the cooking.
Using the back of a knife or a spoon carefully cracked the shells of the eggs and gently peel, if they resist, peel under running water. Dry eggs and set on top of the spinach. You can make a small indentation in the spinach and place the eggs in them. Sprinkle most of the cheese over the eggs and spinach, reserve a small amount to top the eggs with.
In a sauce pan melt the butter then add the flour stir to combine all the raw flour and cook for about one minute. Add the warm ½&½ and whisk while bringing the mixture up to a boil. Once thickened add the salt & pepper and Parmesan whisking briskly to prevent the cheese from lumping together. Allow the mixture to cool a little, add the egg yolk and whisk very briskly to prevent the yolk from scrambling, you can omit the yolk if you want. Taste and adjust seasoning if necessary.
Mollet Eggs Florentine Assembly:
With the bed of spinach and the eggs nestled in, carefully spoon the Mornay sauce over the eggs and spinach and top the eggs with the remaining cheese.
Place under a broiler, but do not have the dish too close to the heat. You want it far enough away so as to allow the eggs to heat up before the cheese and sauce brown too much.