1small serrano or bird pepper, seeded and thinly sliced
1 1/2tsp.kosher salt
1cthinly sliced white onion
Slice fennel bulb thinly, reserving fronds. Chop fronds to equal 1 Tbsp. Fill a large bowl halfway with ice and water.
Bring 1 Tbsp. kosher salt and 2 qt. water to a boil in a Dutch oven over high heat. Remove from heat; add shrimp, and let stand, stirring once, 1 minute or just until shrimp turn pink.
Transfer shrimp to ice water, using a slotted spoon. Reserve 2 cups hot cooking liquid in a medium bowl. Let shrimp stand 10 minutes, stirring once.
Transfer shrimp to a paper towel-lined plate, reserving ice water in bowl.
Whisk lemon juice and next 4 ingredients into reserved hot cooking liquid until salt and sugar dissolve.
Place bowl in reserved ice water, and whisk lemon juice mixture until cooled to room temperature (about 10 minutes).
Remove lemon juice mixture from ice water; discard ice water, reserving chilled bowl for shrimp. Stir together onion, fennel slices, chopped fennel fronds, and shrimp in chilled bowl. Pour cooled lemon juice mixture over shrimp mixture.
Cover and chill 1 hour to 2 days. Serve with a slotted spoon.