Cook the pasta in a large saucepan of salted boiling water according to the packet instructions until al dente.
Meanwhile, heat 2 tablespoons of the oil in a large frying pan over a low heat, add the fennel, chilli and garlic and cook, stirring occasionally, for 5–6 minutes until slightly softened. Stir in the crabmeat, increase the heat to high and pour in the vermouth. Boil rapidly for 1–2 minutes until most of the liquid has evaporated, then remove from the heat and stir in the remaining oil and the lemon juice. Season with salt.
Drain the pasta and stir into the sauce with the parsley. Garnish with fennel fronds and serve immediately.