Put the olive oil into a hot, thick-bottomed, high-sided pan. Grate in the garlic and fennel and soften, then add the anchovies and red onion. Saute, but do not brown. Add the salt then lay the red mullet fillets, skin side up, on top. Sprinkle with the lemon zest, and pour in a little white wine to stop everything frying, and to start stewing. Put in a good glug of olive oil, some pepper and the chilli. Turn the heat down and add the juice from ½ the lemon and some fennel tops.
Meanwhile cook the linguine in boiling, salted water until al dente.
Gently toss the linguine and sauce together, check the seasoning and serve with a drizzle of olive oil on top and some more fennel tops.