Boil the beans quickly to tenderize them, then sauté with lemon, garlic, and toasted fennel so they absorb the fragrant seasonings.
Blanch green beans in plenty of well-salted boiling water until bright green in color and just tender-crisp, 2 to 4 minutes. Drain, then pour into a bowl of ice and cold water to stop cooking. When cold, spread beans on a kitchen towel to drain.
Toast fennel seeds in a large heavy frying pan over medium heat until lightly browned and fragrant, 4 to 6 minutes. Pour onto a plate and let cool briefly, then crush with the bottom of a sturdy glass.
Pour oil into pan and heat over medium heat. Add beans and cook, stirring often with tongs, until well coated with oil and heated through, about 5 minutes. Stir in garlic and cook, stirring, until fragrant, about 20 seconds. Stir in crushed fennel seeds and lemon zest and season with salt and pepper.