In a large stockpot, bring 2 gallons water, seafood boil pouches, ¾ cup salt, ½ cup fish sauce, garlic heads, lime halves, and smashed ginger to a boil over medium-high heat. Reduce heat to medium; add turnips and carrots. Simmer until tender, 14 to 18 minutes; remove from heat. Add shrimp and mushrooms; cover and let stand until shrimp are pink and firm, 5 to 7 minutes. Using a spider strainer, transfer shrimp, mushrooms, carrots, and turnips to a very large bowl.
In a large Dutch oven, melt butter over medium-high heat. Add chopped garlic, green onion, and chopped ginger; cook, stirring frequently, until vegetables are tender, 6 to 8 minutes. Remove from heat; stir in lime juice, cilantro, red pepper, remaining 1 tablespoon fish sauce, and remaining 2¾ teaspoons salt. Add shrimp mixture to butter mixture, stirring until heated through, 2 to 3 minutes. Serve immediately with lime wedges. Garnish with cilantro, if desired.
Notes
*We used Zatarain's Crawfish, Shrimp, and Crab Boil in Bag
Recipe by Taste of the South at https://www.tasteofthesouthmagazine.com/vietnamese-style-shrimp-boil/