4 medium fluffy potatoes (approx 350g), such as Maris Piper or King Edward
1½ tablespoons olive oil
1 large onion, peeled and finely chopped
2 garlic cloves, peeled and finely chopped
250g chestnut mushrooms, cleaned and sliced
200g fresh spinach leaves
2 cooked chicken breasts, finely shredded
3 tablespoons freshly chopped parsley