Bring a small saucepan of lightly salted water to the boil and trim the green beans. When the water is boiling add the beans and cook for 3–4 minutes until just tender, then drain and cool under running cold water.
Meanwhile, dice the flesh of the avocado and toss in the lemon juice to prevent it from turning brown.
Place the mesclun or mixed leaf salad in a large bowl. Add the chicken, pepper, walnuts, beans and avocado, then gently toss until well combined.
Heap the salad onto serving plates and sprinkle over the shallot, if using, and tarragon. Season with a little salt and plenty of pepper, then drizzle over the walnut oil. Serve immediately with lemon wedges and nutty granary bread on the side.