In a large, lidded skillet, toast the orzo over medium heat, stirring occasionally, until nicely browned. Transfer to a plate.
Add the oil and onion to the pan and cook gently for about 5 minutes, stirring often, until onion is softened but not colored. Stir in the garlic and cook for a minute more, until fragrant.
Add the green beans, ¾ teaspoon salt and a good grind of pepper, stirring to coat in the oil, and fry for about 3 minutes. Pour in the stock and bring to a gentle simmer. Once bubbling, cover with the lid, turn the heat down to medium-low and leave to cook, undisturbed, for 10 minutes.
Remove the lid and stir in the orzo and another ½ teaspoon salt. Replace the lid and leave to cook, undisturbed, for another 15 to 20 minutes, until the green beans are very soft and the orzo is cooked through. (It will still be slightly wet.) Set aside to cool for 5 minutes, then stir in the lemon juice and the herbs.
Transfer half the green bean mixture to a large shallow platter, top with half the feta, then repeat. Sprinkle with the lemon zest, a final drizzle of oil and more herbs to taste.