1. Preheat the oven to 220°C, gas mark 7. Peel all the vegetables and cut into large chunks. Place all but the sweet potato in a pan of cold water, cover and bring to the boil. Simmer for 5 minutes, then drain. Place 3 tbsp of the oil in a large roasting tin and heat in the oven until smoking. Add all the vegetables and baste with the hot oil. Season and add the thyme sprigs. Roast for 25 minutes then turn the vegetables and return to the oven for a further 20–25 minutes.
2. While the vegetables are cooking, make the baby Yorkshires. Sift the flour and seasoning into a bowl. Whisk in the egg and milk for a smooth batter. Divide the remaining oil and sliced onion between the 12 holes of a mini muffin tin. Cook for 5 minutes until the onion is golden then pour in the batter and return to the oven for 12–15 minutes until puffy and golden.
3. Mix the soured cream, horseradish and chives and serve with the roasted vegetables and mini Yorkshires.