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1
Set a wide, shallow casserole over medium heat. Add the lardons and render them, turning occasionally, until golden and the fat runs freely. Remove with a slotted spoon and set aside; leave the rendered fat in the pan.
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2
Add the pearl onions and garlic cloves for the pot to the rendered fat. Sweat over medium-low heat, stirring occasionally, until the onions are translucent and glossy but not coloured.
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3
Return the lardons to the casserole. Arrange the fish darnes in a single layer over the onions and lardons. Pour the full bottle of white Burgundy over the fish. Tuck in the bouquet garni. Cover and bring the liquid slowly up to 80–85 °C — a bare, trembling quiver at the surface, never a rolling boil. This is where the dish lives or dies: sustained heat above this point will break the flesh apart and cloud the broth beyond recovery. Maintain this gentle poach for 20–25 minutes.
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4
Lift the fish carefully onto a warm platter and cover loosely. Discard the bouquet garni. Raise the heat and reduce the broth by one third; it should smell concentrated, wine-forward, and faintly of the river.
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5
Knead together 25 g softened butter and 25 g plain flour until a smooth paste forms — the beurre manié. Whisk it into the reduced broth in small pieces over medium heat. Continue whisking until the sauce is smooth, glossy, and lightly coats the back of a spoon without heaviness.
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6
Remove the casserole from the heat. Stir in the crème fraîche and taste for seasoning. The sauce should be rich and round, with no raw flour taste.
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7
Fry the bread slices in butter until golden and crisp on both sides. While still hot, rub each croûton firmly with the reserved raw garlic cloves so that the garlic melts into the surface.
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8
Return the fish to the casserole or pour the sauce over the fish on a warm platter. Arrange the garlic croûtons around the platter in a crown. Serve immediately from the casserole; the fish should hold together and yield cleanly to the touch without falling apart.