Finely chop the mushrooms and sauté them in butter. Set aside 3 mushrooms for decoration and chop the rest. Set aside.
Make a roux with the butter and flour.
Off the heat, add the lukewarm milk, salt, and pepper, stirring constantly.
The sauce will slowly thicken. Add the grated cheese, mustard, and nutmeg.
Continue stirring until the cheese has completely melted. Add the mushrooms and herbs and set aside warm.
Cut the éclairs in half. Fill with the mixture using a piping bag, close, and decorate the top with mushroom slices.
For a better result, prepare your éclairs in advance and refrigerate them. Reheat for 1 minute in the microwave before serving.