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1
Bring a pot of salted water to a boil.
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2
Cut the zucchini lengthwise down the 4 sides, taking off all of the green and about 1/2 inch of the white in each of the 4 pieces.
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3
Discard the white, inner section.
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4
Cut the rest into 1/2-inch dice.
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5
Blanch until crisp-tender, about 3 minutes.
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6
Drain, plunge into ice water, and drain again.
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7
Set aside.
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8
Take off the outer 2 layers of fennel and save the rest for another use.
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9
Trim away any thin, flexible sections near the edge of the fennel.
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10
Cut the firm, thick part into 1/2-inch dice.
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11
Blanch as with the zucchini.
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12
Scrape all pulp and pith from the preserved lemon and cut the yellow rind into fine julienne.
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13
Blanch for 1 minute, refresh in ice water, drain again, and set aside.
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14
(The recipe can be made to this point up to several hours ahead.)
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15
Put 2 cups of nage and the mussel stock in a small saucepan and bring to a boil.
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16
Add the saffron and boil for 2 to 3 minutes.
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17
Set aside.
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18
Bring 3 cups of nage to a boil in a 10-inch-wide pot.
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19
Season both sides of the halibut with salt and pepper.
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20
Add the halibut to the pot and adjust the heat so the liquid just simmers.
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21
Poach until a metal skewer inserted into the center of the halibut meets only a little resistance, 5 to 6 minutes, and the skewer, when left in the fish for 5 seconds, feels barely warm when touched to your lip.
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22
The halibut will be rare (thinner steaks will be ready sooner).
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23
Take the steaks out of the liquid as soon as they are done.
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24
Meanwhile, put the zucchini, fennel, preserved lemon, tomatoes, and shallot in a medium saucepan over medium-low heat.
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25
Add the capers and the olive oil.
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26
Season with salt and pepper.
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27
Just warm the vegetables through, about 2 minutes; do not let them cook further.
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28
To serve, cut the 4 basil leaves across into thin strips.
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29
Stir the olives into the vegetables and squeeze the 1/2 lemon over them.
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30
Pull the skin off the halibut.
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31
Reheat the saffron broth, if necessary.
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32
Put the halibut in the center of the plates and arrange the vegetables over and around the steaks.
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33
Pour the saffron broth around the halibut.
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34
Scatter the basil over and around the fish and serve immediately.