Preheat oven to 180°C (350°F). Bring a pot of water to a simmer, add green beans, and cook for about 15 minutes until just tender. Drain and set aside.
While the beans cook, heat a large skillet over medium heat and brown the chicken in batches for 6–7 minutes per side, until golden and cooked through. Season with salt and pepper and transfer to an oven-safe casserole dish.
Add a little more oil to the same pan and sauté the mushrooms until golden. Season and add them to the casserole with the chicken.
In the same pan, sauté the onions and garlic until softened. (You can blend or finely chop them if you prefer a smoother texture.)
Add a bit more oil, then stir in the risotto rice and thyme. Toast for about 1 minute, then pour in 2 cups of hot stock to deglaze the pan, scraping up any browned bits.
Arrange the green beans over the chicken, then pour the onion, rice, and stock mixture evenly over the top.
Pour in the remaining hot stock and gently pat the mixture to make an even layer. It will look runny—don’t worry, the rice will absorb it while baking.
Cover tightly with foil and bake on the middle rack of your oven for about 40 minutes, until the rice is tender and creamy.
Remove from the oven, stir in the grated cheese and parsley (if using), and serve warm.