Fill a large pot ¾ full of water, add 2 tsp kosher salt, and bring to a rapid boil.
Clean the beans by removing the stem and any brown ends.
Prepare a large bowl of ice water and have it nearby for blanching (ice bath) the beans after they are finished cooking.
Drop the beans into the rapidly boiling water and cook for 2 or 3 minutes or until just tender. You should test for doneness as they cook by taking one out of the boiling water, using some tongs, and dropping them in the ice bath, and then tasting. They should be just cooked, not crisp – but not mushy. Perfectly in-between those two states.
Quickly pull the beans out of the boiling water using tongs or strain them into a colander and then immediately transfer them into the ice bath. Blanching stops the cooking and preserves the bright green color.
After they are completely cool, strain them into a colander set into the sink. Shake off excess water, and then toss them onto some dry paper towels laid out on the counter and dry them off a bit more.
Place them back into a mixing bowl. Pour the melted butter over the beans and gently toss to coat. The beans can be refrigerated at this point.
Before serving, warm them gently on the stovetop, in the microwave, or in the oven. Then, sprinkle with some fine quality flake sea salt, and freshly ground black pepper. Garnish with pickled onion.