Heat a tiny drizzle of the oil in a large saute pan. Season the chicken all over with salt and pepper and brown really well over a high heat until golden – this should take about 2 to 3 minutes. Remove from the pan and set aside.
Add the rest of the oil to the pan, reduce the heat to medium and gently fry the onion, garlic, thyme and zucchini for 8 minutes until the onion is just softened and the zucchini has started to brown.
Add the potatoes, tomatoes and stock, then return the chicken to the pan and simmer, partially covered, for 10 minutes until the potatoes are tender and the chicken is cooked through.
Add the green beans and lemon zest and cook for 2 minutes more until the beans are just tender. Stir in a squeeze of lemon juice and serve.