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1
Roll the chosen dough to fit a 28–30 cm tart tin. Line the tin and trim the edges. If using pâte levée, leave to proof in the tin for 15 minutes; the dough will rise slightly and give the traditional winstub texture. The base is ready when it has visibly puffed and feels lightly springy to the touch.
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2
Melt the lard or butter in a wide, heavy pan over moderate heat. Add the onions and cook, stirring occasionally, for 30–40 minutes. This step is where the tarte lives or dies: the onions must reach a uniform golden colour — sweet, yielding, and fragrant — without darkening to brown. They should offer no resistance when pressed between the fingers and smell of caramel and savoury depth, not of raw sulphur.
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3
Scatter the caraway seeds over the onions and stir through for the final 5 minutes of cooking. Stir in the tablespoon of flour and cook for a further minute until no raw flour is visible.
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4
Remove the pan from the heat. Beat together the crème fraîche, eggs, salt, pepper, and a generous grating of nutmeg. Pour the custard mixture into the onions and stir to combine. The mixture should be homogeneous and just fluid enough to spread without running.
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5
Heat the oven to 200 °C. Spread the onion-custard mixture evenly over the prepared base. If using lardons fumés, lay the strips across the surface. Bake for 25–30 minutes. The tarte is done when the surface carries golden-brown patches and the custard, tested by gentle pressure at the centre, no longer moves in a liquid wave beneath the crust.
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6
Remove from the oven and rest on a rack until warm — not hot. At warm room temperature the custard reaches its silkiest texture. Cut into generous wedges and serve with a green salad dressed in vinaigrette.