Sauce Mornay — Gruyère and Egg Yolk Béchamel

Ingredients

• For the béchamel base
• 30 g unsalted butter
• 30 g plain flour
• 500 ml whole milk, scalded
• 1 tsp fine salt
• 1 pinch white pepper, freshly ground
• For the enrichment
• 2 egg yolks, at room temperature
• 90 g Gruyère, finely grated to order — not pre-grated
• 30 g Parmigiano-Reggiano, finely grated to order — not pre-grated

Method

1. 1
   Melt the butter in a heavy saucepan over medium heat. Add the flour all at once and stir constantly with a wooden spoon or whisk, keeping the mixture moving across the base of the pan. Cook the roux for a minimum of 3 minutes: it should turn a pale straw colour and smell of warm biscuit, not raw paste. The roux is ready when it no longer smells of flour.
2. 2
   Add the scalded milk in a thin, steady stream, whisking vigorously as it goes in to prevent lumps. Once all the milk is incorporated, return the pan to medium heat and continue to stir until the sauce thickens and coats the back of a spoon — approximately 5 minutes. Draw a finger through the coating on the spoon; the line should hold cleanly. Remove from direct heat.
3. 3
   Place the egg yolks in a small bowl. Ladle one large spoonful of the hot béchamel into the yolks, whisking immediately and continuously to raise their temperature gradually. Add a second spoonful and whisk again. Pour the tempered yolk mixture back into the saucepan in a thin stream, whisking the sauce throughout. The sauce should remain smooth and glossy — no visible threads or curds of cooked egg.
4. 4
   This is where the dish lives or dies. Confirm the sauce is off direct heat — or over the barest possible flame, well below 70°C. Add the grated Gruyère and Parmigiano in three or four separate handfuls, stirring each addition in slow, deliberate circles until it has melted completely and the sauce is smooth before the next handful goes in. Adding cheese over high heat contracts the casein proteins and expels fat, producing a grainy, oily sauce that cannot be recovered. The finished sauce should pull away from the spoon in slow, heavy sheets.
5. 5
   Taste for salt and white pepper. The sauce should register clearly as cheese — nutty, saline, rounded — with no background taste of raw flour or scorched milk. Adjust seasoning if needed. Use immediately, or press plastic wrap directly onto the surface and hold at 60°C until required.
6. 6
   To gratin: pour the Mornay over blanched vegetables, fish, or pasta arranged in a gratin dish. The sauce layer should measure 8–10 mm. Flash under a preheated salamander or broiler at maximum heat for 2–3 minutes. The surface should blister and colour unevenly in mahogany patches; the sauce beneath should remain fluid when the dish is tilted. Remove immediately — prolonged heat breaks the sauce.