Bring a large pot of water to a rapid boil. Add a generous pinch of salt, then add the green beans. Cook for about 4 minutes, until the beans are tender and bright green in color. Drain into a colander and set aside.

While the green beans are cooking, set a large skillet over medium heat. When hot, add the butter, walnuts, lemon zest and juice, red pepper flakes if using, and another pinch of kosher salt. Toast the mixture, stirring frequently, for 3-4 minutes, until the butter is lightly browned and the walnuts are fragrant and golden. Watch carefully that the walnuts don’t burn.

When walnuts are toasted, add green beans to the skillet, followed by dollops of the pesto. Toss to warm through and lightly coat the green beans. Top with extra pesto to taste, and serve right away.