1. Bring a medium pot of water to boil and blanch green beans until tender-crisp, 2 to 3 minutes (don't overcook... you don't want limp green beans). Drain and put in a large heatproof bowl.
2. Bring 2 1/2 cups of water to boil in another medium pot with vinegar, sugar, salt, chiles, peppercorns, and seaweed. As soon as the liquid boils, pour over beans and add the lemon slices. Let cool to room temperature. Slit 3 of the chiles to let seeds spill into liquid. Chill, covered, overnight to let flavors develop. Serve cold.