Preheat the oven to 220 degrees C. Scrub or peel the root vegetables, according to their type and your taste. Halve or quarter large potatoes. Cut large carrots or parsnips in half lengthways, then cut the pieces across in half again. Cut sweet potato, swede or kohlrabi into large chunks (about the same size as the potatoes). Leave shallots or pickling onions whole.
Place all the vegetables in a large saucepan and pour in enough boiling water to cover them. Bring the water back to the boil, then reduce the heat and simmer for 5–7 minutes or until the vegetables are lightly cooked, but not yet tender.
Drain the vegetables and place them in a roasting pan. Brush the vegetables with olive oil and sprinkle with cracked black peppercorns. Add the herb sprigs to the pan and place the pan in the oven.
Roast for 30–35 minutes or until the vegetables are golden-brown, crisp and tender. Turn the vegetables over halfway through cooking time. Serve hot, garnished with extra sprigs of thyme or rosemary, if desired.