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1
Bring a pan of water to the boil. Score the base of each tomato with a shallow cross, blanch for 10 seconds, then transfer immediately to an ice bath. When cold, slip off the skins, halve each tomato crosswise, squeeze gently to expel the seeds, and cut the flesh into 1 cm dice. The concassée is ready when the pieces are clean-edged and free of seed pockets.
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2
Warm the olive oil in a wide sauté pan over medium heat. Add the onion and sweat, stirring occasionally, for 5–6 minutes. Add the minced garlic and continue to cook until both are soft and translucent. This step lives or dies on one condition: the garlic must not colour. Browning produces acrolein, an irreversibly bitter compound that no quantity of tomato will mask. The pan should smell of sweet allium, not of toast.
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3
If using tomato paste, add it now and stir constantly for 2 minutes until it deepens to a brick-red. Add the tomato concassée, season with salt and a pinch of sugar if the tomatoes taste sharp, and nestle in the bouquet garni. The pan will hiss and the tomato liquid will release almost at once.
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4
Simmer uncovered for 25–30 minutes, stirring occasionally. The tomatoes should break down partially but retain visible 1 cm pieces throughout. The sauce is not a purée; if it begins to look entirely smooth, remove it from the heat immediately.
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5
Pour in the 200 ml of completed sauce tomate or demi-glace and stir to combine. Simmer for a further 10 minutes. The sauce is ready when it mounds on a spoon held level and spreads slowly when the spoon is tilted — thick enough to coat but fluid enough to flow.
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6
Remove and discard the bouquet garni. Taste and adjust the seasoning. The sauce should be assertive — garlic-forward, with the brightness of fresh tomato and the savour of the added stock base.