Have ready a large bowl of ice water. In a 2-quart saucepan, combine the sun-dried tomatoes, lemongrass, 2 tsp. salt, and 1-3/4 cups water. Bring to a boil over medium-high heat, stirring to dissolve the salt. Transfer to a medium bowl and set over the bowl of ice water. Stir frequently until chilled, 10 to 15 minutes. Put 1 halibut fillet in each bag. Divide the broth (including the tomatoes and lemongrass) among the 4 bags. Seal the bags, pressing out as much air as possible, and gently massage the fish to coat it all over. Refrigerate for 2 to 4 hours to season the fish.
Meanwhile, put the basil, the 3 whole cloves garlic, and 1/4 tsp. salt in a food processor and pulse to a coarse paste. With the motor running, slowly pour 1/4 cup of the olive oil through the feed tube and process until emulsified.
Fill a 4-quart pot with 3 inches of water and clip a deep-fat, probe, or instant-read thermometer to the side of the pot. Heat the water slowly over low heat to 140°F, about 15 minutes. Put the bags of fish in the water (the tops of the bags can stick out) and cook, maintaining a water temperature of 135°F to 145°F, until the center of the fish registers 130°F to 135°F on an instant-read thermometer (open the bags to check), 12 to 15 minutes. Transfer the bags to a rimmed baking sheet and let sit for 5 minutes.
Meanwhile, heat the remaining 2 Tbs. olive oil in a 2-quart saucepan over medium heat. Add the minced garlic and cook, stirring, until fragrant, about 1 minute. Add the zucchini and 1/4 tsp. salt and cook, stirring often, until the zucchini is barely tender, 3 to 5 minutes. Season to taste with salt and pepper.
Divide the zucchini among 4 dinner plates. Using scissors, snip off one corner of each bag and drain and discard the liquid. Open each bag and cut down the center perpendicular to the seal to expose the fish. Using a spatula, lift the fish from the bags and place on the plate with the zucchini. Discard the lemongrass; rinse and thinly slice the tomatoes crosswise. Serve the fish and the zucchini with the pesto and tomatoes.
You can prepare the pesto up to 4 hours ahead; cover with plastic wrap pressed directly onto its surface.