Easy chicken casserole

Name: Easy chicken casserole
Prep time: 15 min
Cook time: 45 min
Total time: 1 h
Cuisine: French
Ingredients: 1 tbsp. olive oil; 1 chicken pieces (eight in total), such as thighs and drumsticks; 1 onion, finely sliced; 2 garlic cloves, crushed; 2 celery sticks, thickly sliced; 2 carrots, cut into 2cm chunks; 2 leeks, cut into 2cm chunks; 600 g potatoes, peeled and cut into large chunks; 1 level tbsp plain flour; 600 mL hot chicken stock; 2 rosemary sprigs; half a lemon