Boil potatoes in a saucepan for seven minutes.
Add carrots and boil for eight more minutes until tender, then drain.
Melt butter in a large saucepan and fry leeks and bacon until soft and golden.
Preheat the grill to a hot temperature.
Stir flour into the bacon mixture and cook for one minute.
Gradually whisk in apple juice and milk, simmering for two minutes.
Add vegetables to the sauce, coating well, and season with salt and pepper.
Transfer to a 28x22cm baking dish and sprinkle with thyme and olive oil.
Grill for 10 minutes until golden and hot.