In a saucepan of boiling water, boil the potatoes for seven minutes, or until half cooked. Add the carrots and continue to boil for a further 7-8 minutes, or until both the carrots and potatoes are tender. Remove from the heat and drain.
Meanwhile, heat a large saucepan over a medium heat. Add the butter and, once melted, gently fry the leek and bacon together for 7-8 minutes, or until the leeks are soft and the bacon golden-brown.
Preheat the grill to hot.
Sprinkle the flour into the pan with the bacon, stir and cook on the heat for one minute. Gradually add the apple juice and milk, stirring as you add until you have a smooth sauce. Bring to a simmer and simmer for two minutes.
Add the cooked potatoes and carrots to the pan and mix well so that everything is coated in the sauce. Taste and season with salt and pepper to taste.
Tip into a medium ovenproof baking dish (approximately 28x22cm/11x8½in in size). Sprinkle generously with fresh thyme leaves and a drizzle of olive oil. Place under the preheated grill for 10 minutes, or until golden and piping hot.