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1
Combine 250 ml water, 100 g butter, and a pinch of salt in a heavy saucepan. Bring to a rolling boil, ensuring the butter is fully melted before the water reduces. Remove from heat and add 150 g flour all at once. Beat vigorously with a wooden spoon until the mixture forms a smooth, cohesive ball. Return to medium heat and stir continuously for 2–3 minutes to dry the paste; the paste is sufficiently dried when a thin, dry film forms on the bottom of the pan.
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2
Transfer the dried paste to a bowl. Beat in 4 eggs one at a time, incorporating each fully before adding the next. This is where the dish lives or dies: eggs added too quickly prevent proper emulsification, producing a heavy, broken paste rather than one that is smooth, glossy, and holds a soft peak when lifted on the spoon.
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3
Beat 80 g finely grated Gruyère into the finished paste until evenly distributed.
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4
Bring a wide pot of salted water to a gentle simmer — not a rolling boil, which tears the gnocchi apart. Using two teaspoons dipped in hot water, shape quenelle-sized portions and drop them into the simmering water in batches of 8–10. The gnocchi will sink, then rise to the surface after 3–4 minutes. Continue cooking for a further 2 minutes after they float. Remove with a slotted spoon and drain on a clean cloth.
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5
Arrange the poached gnocchi in a single layer in a buttered gratin dish. Nappé generously with sauce Mornay — béchamel enriched with Gruyère and egg yolks. Scatter additional grated Gruyère over the surface.
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6
Gratinée under a hot grill or in a 220 °C oven for 10–12 minutes. The dish is ready when the sauce bubbles fiercely across its entire surface and the top is spotted with deep golden-brown. Serve immediately.