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1
Heat the oven to 220 °C. Combine the coarse sea salt, fennel seeds, and dried herbes de Provence in a large bowl. Add the egg whites and work together with the hands until the mixture coheres when a fistful is squeezed — it should hold its shape like damp sand.
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2
Pack the cavity of the unscaled fish with the fennel fronds, thyme, bay leaves, and lemon slices. Add the splash of pastis if using. The cavity should be full but the fish still capable of lying flat.
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3
Spread a 1 cm bed of salt mixture across a baking tray large enough to hold the fish. Lay the fish on it. This is where the dish lives or dies: cover the fish completely with the remaining salt mixture, pressing firmly at every point to eliminate air gaps and seal the crust hermetically — any breach will allow steam to escape and the flesh to dry. The finished shell should be uniformly smooth with no cracks or thin patches.
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4
Bake at 220 °C for 25–30 minutes per kilogram of fish weight. The crust will harden to a ceramic-like shell and take on a light golden colour. An instant-read thermometer inserted through the crust at the spine should read exactly 58 °C for perfectly moist flesh; do not allow the temperature to exceed 60 °C.
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5
Remove the tray from the oven and allow the fish to rest, undisturbed and still sealed in its crust, for 5 minutes. This allows carryover heat to finish the cooking evenly throughout the flesh.
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6
At the table, score around the equator of the crust with a serrated knife, then lift the top shell away in one piece to reveal the fish beneath. Peel back the skin — it will come away cleanly — and serve the flesh directly from the tray with a drizzle of olive oil and a squeeze of lemon. The flesh should be white, just set, and gleaming with retained moisture.