Heat olive oil in a large skillet over medium-high heat. Add green beans and toss to evenly coat with olive oil. Cook, stirring occasionally for even cooking, until softened, about 5 minutes. Add walnuts, white balsamic vinegar, salt, and pepper; mix to combine. Cook, stirring occasionally so walnuts don’t burn, until beans are soft but maintain some crunch, 5 to 10 minutes.
Remove from heat and sprinkle with lemon juice and red pepper flakes.