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1
Combine the lemon juice, zest, sugar, whole eggs, and egg yolks in a heavy saucepan. Whisk briefly to amalgamate before any heat is applied. The mixture should appear uniform and pale yellow.
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2
Set the pan over medium-low heat and cook, whisking continuously, until the curd thickens — approximately 82 °C. Do not allow the mixture to boil; boiling scrambles the eggs. The curd is ready when it coats the back of a spoon thickly and a finger drawn across the spoon leaves a clean, stable line.
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3
Remove the pan immediately from the heat. Add the cold butter in pieces and whisk continuously — or blend with a stick blender — until every piece is fully incorporated. This is where the dish lives or dies: the curd must be at maximum temperature and still fully fluid at the moment the butter enters it; only then can full emulsification occur, producing the characteristic silk texture. Cold curd with butter produces a greasy, separated finish; hot curd with cold butter, whisked without pause, produces silk.
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4
Taste the curd before straining. It should register simultaneously as sharply bright from the lemon juice, sweet without cloying, and richly round from the butter and yolks. If too sharp, whisk in an additional 10 g sugar. If too sweet, add 15 ml more lemon juice. The balance is everything in this preparation.
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5
Pass the warm curd through a fine sieve directly into the blind-baked, still-warm shell. The sieve removes zest fibres and any coagulated egg solids in a single step. The surface should be level, even, and glossy.
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6
Allow the tart to set at room temperature for 1 hour, then refrigerate. The finished filling should be deep, glossy, and an even yellow — the colour of a good Hollandaise. When the tart is tilted gently, a correctly set curd does not flow; it holds its shape cleanly where it meets the shell.