Lobster Salad with New Potatoes and Pickled Onion

Bring beer to a boil in a large heavy pot. Add lobsters, cover, and steam until shells are bright red and meat is cooked through, 10 –12 minutes. Remove from pot with tongs and let cool 10 minutes. Crack shells; remove meat from tail, claws, and knuckles. Meanwhile, place potatoes in a large saucepan and add water to cover by 2"; season with salt. Bring to a boil and cook until tender, 10 –12 minutes; drain. Transfer potatoes to a large bowl and lightly crush with a fork. Bring vinegar, sugar, 1 tablespoon salt, and 1/2 cup water to a simmer in a small saucepan. Add onion, remove from heat, and let sit 5 minutes; drain. Let pickled onion cool. Whisk lemon juice and 1/4 cup oil in a medium bowl; season with salt. Add mint, pickled onion, potatoes, and lobster meat; toss to coat. Add frisée, if using, and toss to combine. Drizzle with more oil; season with pepper and more lemon juice, if desired. DO AHEAD: Lobsters can be cooked and meat removed 2 days ahead; cover meat and chill. Onions can be pickled 1 day ahead; cover and chill. Potatoes can be cooked 1 day ahead; cover and chill. Bring potatoes to room temperature before using.