Preheat the oven to 200C/400F (fan).
Heat the oil in a large frying pan. Add the onion, garlic, and carrots and cook on medium heat, stirring regularly until the onion softens.
Add the wine, bring to the boil, then simmer for 2-3 minutes. Add the cream and mustard and stir together. Heat through until almost boiling, then stir in both cheeses, along with a good pinch of salt, pepper, and dried thyme.
Add the cooked chicken and spinach and heat through until the spinach wilts.
Spoon the chicken mixture into a large baking dish, then layer the potatoes on top, starting from the outside and working inwards. Once covered in the potato slices, brush with the melted butter and sprinkle on a pinch of salt and pepper.
Place in the oven to bake for 30-40 mins, until the potatoes are tender and golden brown.
Top with fresh thyme before serving.
