Remove the skin and fat from the duck breasts. With a sharp knife, score the flesh on both sides in a crisscross pattern. Put the breasts in a bowl and add two-thirds of the garlic, the lemon juice, half of the thyme sprigs, the rosemary and 1 tbsp of the oil. Turn the breasts to coat with the flavourings, then leave to marinate while you prepare the rest of the ingredients.
Put the buckwheat in a heavy frying pan and toast over a medium heat, stirring and tossing, for 4–5 minutes or until it has become slightly darker in colour. Remove from the heat.
Bring the stock and water to the boil in a saucepan. Add the toasted buckwheat and return to the boil, then reduce the heat, cover and cook for 10–15 minutes or until the stock has been absorbed and the buckwheat is tender. Remove from the heat and set aside, still covered, until needed.
Heat a ridged chargrill pan over a medium–high heat for 5 minutes. Meanwhile, blanch the green beans for 1–2 minutes. Drain and refresh under cold running water. Cut the beans in half and put in a salad bowl. Add the salad leaves, basil, red onion and olives, if using, and toss to mix.
Remove the duck breasts from the marinade and place on the hot chargrill pan. Cook for 3 minutes on each side for rare (cook longer if you prefer it well done). Remove the duck to a board. Place the zucchini, spring onions and tomatoes on the pan and cook for 1–2 minutes or until lightly charred all over.
Combine the remaining garlic and oil with the vinegar in a small bowl. Add the leaves from the remaining thyme sprigs. Whisk together, then drizzle over the salad. Spoon the buckwheat over the top, then arrange the zucchini, spring onions and tomatoes over it. Slice the duck breasts, place over the salad and serve.