To cook beans: Generously salt boiling water and add green beans; cook until crisp-tender, 4 to 5 minutes. Drain well in a colander and pat dry with paper towels, then transfer warm beans to bowl with lemon zest and juice. Using tongs, toss until well coated. Taste and adjust seasoning with salt and pepper.
To serve: Arrange green beans on a serving platter and top with toasted hazelnuts; serve warm.
