Ready a stacked-style bamboo steamer. You'll need two of the layers, one for steaming the onions and fennel soft, and another for the herbs and fish.
PREP THE FISH
Trim the fish per step by step below.
Transfer to a plate and place in refrigerator, uncovered, for a moment. Alternatively, you can put the fish in a baking dish and cover with plenty of ice to, you know, ready the mood.
FILL AND STACK YOUR STEAMER
Fill a Dutch oven (large enough to house your steamer) about two-thirds full with water. Throw in the mint and lemon slices.
Bring to a raging boil, then taper to a steady, steamy boil.
For the first bamboo steamer layer, lay down a bed of trimmed green onions as if to build a raft.
Place the fish on top of the onions.
In a small bowl, toss together the chopped tarragon, the parsley, the fennel fronds, and the thyme leaves.
Sprinkle in an even layer on the top side of the fish.
Sprinkle with the black pepper.
Top the fish with another bamboo layer, and then add the shaved fennel bulb and the sweet onion to this top steamer.
STEAM IT ALL! Cover the steamer stack, place over the simmering water, and steam for about 20 minutes before checking the fish. Depending on the size of the fish and the intensity of your steam, this can take anywhere from 20 to 35 minutes.
Check the doneness of the onions and fennel. They should be soft after about 20 minutes of cooking time.
When tender to the tooth, transfer to a blender and add the capers, oil, lemon juice, red wine vinegar, and Tabasco. Blend until silky smooth.
In the steamer layer that contained the onions and fennel, add the arugula, then cover and return the top steamer layer to the steamer stack.
Steam 15 minutes, letting the arugula wilt while the fish is finding its way to completion. Turn off the heat.
SERVE YOUR BEAUTIFUL FISH
Find your most majestic serving platter--one that has plenty of depth for sauce.
Once the fish is cooked through, pour half of the fennel sauce onto the base of the platter.
Layer the green onions over the sauce.
Now the fish. Then, the rest of the sauce. Sprinkle with 3/8 teaspoon of the salt.
Top with the arugula. Hit it with the final 1/8 teaspoon of salt.
Laboriously remove the lemons from the hot water. Place them indiscriminately around, on, and about the fish.
Use tablespoons and forks to serve the fish.
Step by Step: Trimming a Whole Fish 1) Using a pair of kitchen scissors, trim off the dorsal and pelvic fins. 2) Holding the edge of the pectoral fins, trim as closely to the fish's body as possible. 3) Move the fish to a cutting board, and score at a sharp angle all the way to the backbone, every inch or so.