In a large pot over medium heat, boil the potatoes until slightly tender but not fully cooked, about 10-12 minutes. Drain and set aside.
Season the chicken with salt and pepper on both sides.
Heat oil in a large skillet over medium heat. Cook the chicken until golden about 4 minutes per side.
Meanwhile, in a mixing bowl, whisk well the milk and flour, until no visible lumps. Stir in the thyme, garlic, chicken stock, lemon juice and zest. Tasteand add salt and pepper, to your taste.
Pour the sauce over the chicken, and make space in the pan for the boiled potatoes, mushrooms and beans.
Cook over medium-low heat, for 10-15 minutes, or until chicken is cooked through and potatoes are super tender.
The sauce will thicken quite fast and nicely coat everything, so make sure you give it a good stir halfway to avoid catching the bottom of the pan.
Garnish with lemon slices and fresh thyme and enjoy!