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1
Heat the oven to 325 degrees F. Rinse the fish under cold water and dry well with paper towels.
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2
In a large, wide bowl, whisk together the lime juice, olive oil, cilantro, a few generous pinches of salt and the pepper. Add the fish to the marinade, turning to coat and getting some inside the cavity. Set aside for about 10 minutes while you prepare the rest of the ingredients.
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3
Halve the fennel bulb and thinly slice one half. Cut the other half into 1/8-inch dice. Do the same with the onion.
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4
Take two pieces of aluminum foil and fold them together so that they form big piece (see photo below). Lay the foil shiny-side down on a large, rimmed baking sheet. Arrange half of the sliced red onion, sliced fennel and lime along the middle of the foil, over an area roughly the same size as the fish.
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5
Remove the fish from the marinade and slather the cavity with the mustard. Then scatter the butter chunks and the chopped onion and fennel inside the cavity, filling it just enough that it looks plump, without the vegetables spilling out all over the place.
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6
Lay the fish gently on top of the sliced vegetables, and then arrange the rest of them on top of the fish, finishing with a row of lime slices. Start to fold up the foil so that liquid won't seep out the sides, and then drizzle the marinade over the top of the fish. Fold and crease the foil into a neat package, leaving a little space between it and the fish. Put the fish (on the baking sheet) in the oven and bake for 45 minutes to an hour, until the fish is cooked through. Serve with boiled herbed potatoes and a salad.
