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1
Bring a large pot of well-salted water to a rolling boil. Cook the rotini until al dente — the pasta should retain a distinct bite at the centre. Drain, reserving 120 ml of pasta water, and set aside. The test: a strand cut through should show a faint white core.
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2
Melt the butter in a wide, heavy pan over medium heat. Add the garlic and cook until softened and fragrant but not coloured. The test: the raw sharpness of the garlic has given way to a sweet, gentle aroma.
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3
Pour in the cream and the half-and-half. Add the Cajun seasoning and black pepper. Reduce over medium heat, stirring occasionally, until the sauce coats the back of a spoon cleanly and a finger drawn across the spoon leaves a clear line. This is where the dish lives or dies: the reduction must be sufficient to hold the pasta without becoming heavy, but the sauce must not be pushed further — over-reduction produces a cloying result that overwhelms the crawfish sweetness.
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4
Add the hot, drained rotini to the pan and toss to coat thoroughly. If the sauce tightens too much, loosen with a splash of the reserved pasta water. The test: each spiral is evenly glazed and the sauce moves fluidly through the pasta rather than pooling at the bottom of the pan.
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5
Add the crawfish tail meat and fold gently into the pasta. Heat only long enough to warm the crawfish through — 60 to 90 seconds over low heat. The test: the tails are heated to temperature but remain tender; pressing one lightly with a fork meets gentle resistance, not rubber.
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6
Taste and adjust seasoning. Serve immediately, directly from the pan. The test: the dominant note is crawfish sweetness, with cream and butter behind it and a mild, background warmth from the Cajun seasoning.