Sift the flour into a large mixing bowl and add the beaten egg and milk and mix to form a batter. Mix in the thyme, salt and pepper and set aside for at least 4 hours. Preheat your oven to 250°C, whilst the oven is heating up place a 25cm skillet in there with the cooking oil to heat up. Bring a large pan of salted water to the boil and add the carrot and parsnip to the boiling water and cook for 10 minutes until they begin to soften. When the vegetables have softened, heat a frying pan that can be transferred to the oven and sear off the vegetables in the butter (including the onions) so they get some colour for 2-3 minutes. Now add in the balsamic vinegar and transfer to the oven for 10 minutes. Remove the vegetables and set aside for a minute or so. Now working as quickly as possible to keep the oven hot pour the batter into the cast iron skillet and then add the vegetables to the batter and drizzle with any left over balsamic in the pan the vegetables were roasted in. Close the oven and roast for 20 minutes. Serve with a sticky balsamic reduction.