Prepare and ice bath by filling a large bowl with a couple of handfuls of ice, top with cold water and set aside.
In a large pot. combine 12 cups (3 quarts) water with 1/2 cup of the kosher salt and bring to a boil over medium-high heat. Add the green beans and blanch until they are bright green and tender but still crunchy, 30 to 45 seconds. Immediately drain the green beans and transfer them to the ice bath. Allow them to cool for a few minutes, then drain again. Refrigerate them while you prepare the other ingredients.
In a small skillet set over medium heat, swirl 1 tablespoon of the olive oil to coat the bottom of the pan. Add the breadcrumbs and stir to coat with the oil. Cook, stirring frequently to toast the crumbs evenly, until they are crispy and brown, 3 to 4 minutes. Remove the pan from the heat, stir in the lemon zest, and set aside.
In a large skillet, heat the remaining 1 tablespoon olive oil over high heat. Once it begins to smoke, add the green beans, the remaining 1/2 teaspoon salt, the garlic, and butter. Cook tossing the beans frequently to coat, until the butter melts and the garlic begins to brown just a bit, about 1 minute. Add the lemon juice and sage, toss once more, and remove the pan from the heat.
Spread the beans on a serving platter and top with the breadcrumbs and a sprinkle of Maldon salt. Serve immediately.
