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1
Make a medium-thickness béchamel: melt the butter in a heavy saucepan over moderate heat, add the flour, and cook the roux, stirring, until it smells faintly biscuity but has taken no colour. Add the warmed milk in a steady stream, whisking constantly. Cook over low heat, stirring, until the sauce coats the back of a spoon and a finger drawn across the spoon leaves a clean line.
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2
Pass the double-concentrate tomato purée through a fine sieve into a small bowl, pressing it through with the back of a spoon. No seeds or fibres should remain in what passes through.
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3
This is where the dish lives or dies: stir the sieved purée into the hot béchamel one tablespoon at a time, checking the colour after each addition. The target is a uniform pale coral-pink — the colour of a winter sunrise — not red. Stop adding purée the moment the sauce is a blush. If the colour tips toward red, the sauce has crossed from Aurore into another sauce entirely.
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4
Remove the pan from the heat. Add the cold butter piece by piece, swirling or stirring until each piece is fully absorbed before adding the next (monter au beurre). The surface of the sauce should carry a visible gloss.
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5
Stir in the cream if using. Season with fine salt, white pepper, and a bare pinch of sugar if the tomato purée is sharp. Taste: the sauce should be gentle and round, with tomato present as warmth rather than as an assertive flavour.