Line a large baking sheet with foil, or ready a large glass baking 9x13 baking dish. Preheat the oven to 350 degrees.
In a large bowl, toss the potatoes, mushrooms, green beans, onion and garlic slices in 2 tbs olive oil. Season with salt and pepper. Arrange the veggies on an even layer on the prepared pan.
Meanwhile, rub each of the thighs with olive oil and salt and pepper. Nestle the chicken thighs on the baking sheet and sprinkle with thyme. Arrange one thin slice of lemon over each of the thighs and squeeze out any remaining juice over the rest of the pan.
Place the pan in the preheated oven and cook for about 35 to 40 minutes, until the chicken is cooked through.
Remove from oven and allow to cool for a few minutes before serving.