1. Place potatoes in a medium saucepan; add enough water to cover by 1 inch. Add 2 tablespoons salt. Bring to a boil, then reduce heat to medium-high and cook until potatoes are easily pierced with the tip of a knife, 15 to 20 minutes. Meanwhile, whisk together pickling liquid, mustard, and sugar. Whisk in oil; season with salt and pepper. Stir in fennel fronds. Transfer potatoes to a bowl with a slotted spoon, and toss with 2 tablespoons dressing.

2. Return water to a boil; add shrimp. Remove from heat and let stand, stirring occasionally, until shrimp are bright pink and opaque, 2 to 3 minutes. With a slotted spoon, transfer to an ice-water bath until cool. Drain well. Toss lettuce and sliced fennel with 2 tablespoons dressing; arrange on a platter. Top with potatoes, shrimp, cornichons, and eggs. Drizzle with remaining dressing; serve.