Thickly slice the onion. Halve the unpeeled potato and cut into 1cm-thick slices. Trim the beans, then halve on an angle. Crush or finely chop 2 garlic cloves. Cut the chicken into quarters.
Step 2 — Soften onion
Heat 1 tbs olive oil in a medium deep frypan over medium heat. Cook the onion, stirring, for 3-4 mins until softened.
Step 3 — Add chicken
Add the chicken, potato, garlic, Tuscan seasoning and 1½ tsp dijon mustard to the onion. Stir to combine, then season with salt and pepper.
Step 4 — Simmer casserole
Add the tomatoes, ½ tsp sugar and 250ml (1 cup) boiling water to the chicken mixture. Cover and bring to the boil over high heat. Reduce the heat to medium and cook, covered, for 25 mins or until the potato is tender. Remove the pan from the heat. Taste, then season with salt and pepper.
Step 5 — Serve up
Meanwhile, bring a medium saucepan of water to the boil. Add the beans and cook for 2-3 mins until tender. Drain and return to the pan. Add 10g butter, season with salt and pepper and toss to coat. Divide the casserole and beans among plates and enjoy!
Step 6 — Make it yours
If you have some on hand, scatter a little grated parmesan over the casserole to serve.