Preheat the oven to 200°C/fan 180°C/gas mark 6. Divide the oil between two large, heavy-based frying pans. Add the potatoes and garlic cloves to one and the beetroot to the other and fry gently over a medium heat for 5-7 minutes until everything begins to soften. Remove the garlic cloves and season to taste.
In a lightly oiled 22cm round ovenproof dish, layer the potatoes and beetroot in sections alternatively with their sides facing upwards. Make sure the veg are tightly packed into the dish. Nestle the garlic cloves in, sprinkle over the thyme and pour over the stock. Bake for 1 hour to 1 hour 15 minutes or until the vegetables are tender and the top is a nice golden crisp brown.