Melt 11/2 oz (40 g) of the butter in a small saucepan and gently soften the onion in it for 10 minutes.
Then stir in the flour, cook for a minute or two and add the milk a little at a time, stirring after each addition, to make a smooth sauce. Taste and season the sauce, and let it cook for 6 minutes over a very gentle heat. Meanwhile boil the eggs by placing them in cold water, bringing them up to the boil, and simmering them for 6 minutes, afterwards cooling them under cold running water.
Now add two-thirds of the grated cheese to the sauce, stir it in and allow to melt; then transfer the sauce to a liquidiser, add the remaining ½ oz (10 g) butter, the cream and the chopped drained spinach, and whiz the sauce until it’s a pale green colour and only very tiny speckles of the spinach are visible. Season the sauce again, adding some nutmeg.
Peel the eggs now, then halve them. Put a layer of sauce in a buttered gratin dish and arrange the eggs on top, rounded side up. Cover with the rest of the sauce, sprinkle with the remainder of the cheese and place the dish under a hot grill until the cheese is brown and the sauce is bubbling.