Scrub 24 oz red potatoes; cut into quarters. Shuck 2 ears of corn, removing any strings; cut each cob crosswise into 4 pieces. Cut 12 oz pkg bratwurst on an angle into ½-inch thick slices. Trim 2 scallions and thinly slice, keeping dark greens separate. Finely chop 2 teaspoons garlic. Zest half of 1 lemon. Rinse 10 oz pkg shrimp and pat dry.

Cut zested lemon in half; into a medium pot, squeeze all of the juice and add lemon halves. Add potatoes, 2(¼ oz) hondashi, 2 packets Cajun seasoning, 4 cups water, and 1 teaspoon salt. Bring to a boil over high heat. Reduce heat to medium-low and simmer, covered, until potatoes are nearly tender, about 10 minutes.

Add sausage and corn to pot. Continue simmering until potatoes are easily pierced with a knife and corn is bright yellow and tender, 5-10 minutes. Stir in shrimp, cover, and remove from heat. Let stand until shrimp are opaque, stirring halfway through, about 5 minutes.

Meanwhile, melt 8 tablespoons butter in a small skillet over medium-low heat until foaming. Add scallion whites and light greens and chopped garlic, cook, stirring occasionally, until softened and aromatic, 1-2 minutes. Add remaining Cajun seasoning and lemon zest, cook until fragrant, about 30 seconds. Transfer to a bowl.

Once shrimp is cooked through, drain all ingredients in pot through a colander. Transfer to a serving platter; sprinkle with scallion dark greens. Cut remaining lemon into wedges. Serve shrimp boil with lemon wedges, Cajun butter, and hot sauce of choice, if desired.