1. Scald the milk (heat over medium until steaming and small bubbles appear around the edge, approximately 180°F / 82°C — do not boil) with the onion slice in a heavy saucepan. Remove and discard the onion.
2. Mix the flour with enough cold water to form a thin, lump-free slurry — approximately 1/4 cup (60 ml) cold water — then whisk it gradually into the hot milk. This is a slurry-thickening technique: the starch granules hydrate in cold water first, preventing lumps when added to hot liquid. Cook over medium-low heat, stirring constantly at first and then frequently, for 20 minutes, until the mixture coats the back of a spoon and no raw-flour taste remains.
3. Meanwhile, combine the canned tomatoes and sugar in a separate saucepan. Cook over medium heat for 15 minutes, stirring occasionally, until the tomatoes have broken down and the mixture has thickened slightly and deepened in color.
4. Remove the tomato mixture from heat. Add the baking soda and stir to combine — the mixture will foam briefly as the soda neutralizes the tomato's acidity. Pass the tomato mixture through a fine-mesh sieve or food mill to remove seeds and skins, pressing firmly with the back of a spoon.
5. Place the butter, salt, and pepper in the bottom of a warmed serving tureen or large heatproof bowl.
6. Combine the hot thickened milk with the sieved tomato mixture, stir briefly, then immediately strain the combined soup through a fine-mesh sieve directly into the tureen over the butter. Stir gently until the butter melts. Serve at once — the soup should be piping hot and uniformly pale coral in color, with no curdled streaks.
7. If the soup curdles on combining (which the baking soda is intended to prevent), it is still safe to eat but the texture will be uneven. Work quickly and ensure both components are hot when combined.
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