Heat olive oil in a skillet over medium heat; add garlic and cook and stir until fragrant, about 2 minutes. Add walnuts to skillet, cook and stir until lightly toasted, about 2 minutes; stir in green beans, toss lightly to coat, and cook and stir until warmed through, 2 to 3 minutes.
Transfer green beans to serving bowl and immediately top with crumbled feta cheese.
