Step 1 Fill a large pot with plenty of water (at least 6 quarts) and bring it to a boil. Add enough salt for it to taste like seawater.
Step 2 Drop in the green beans and cook them until just blanched (tender-crisp), about 4 to 5 minutes.
Step 3Drain the beans and shock them in ice water. Drain again, pat them dry in a towel, and set aside until ready to use. (If overnight, refrigerate.)
Step 4Just before serving, melt the butter in a large skillet. Add the beans and warm through over medium heat. If they're still too crunchy, you can continue warming them, stirring now and then, until they're the texture you like.
Step 5Remove from the heat and stir in the lemon zest and juice. Add fleur de sel or Maldon sea salt and freshly ground pepper to taste, and serve.