Peel and cut the potatoes into halves or quarters, depending on the size. You want big enough chunks.
Peel the carrots and parsnips. If they are big, slice them in halves, otherwise you can leave them whole.
Peel and cut your beetroot into smaller chunks than your potatoes as they take longer to cook and they won't be parboiled.
Preheat the oven to 200C / 400F
Add some water to a big pot and bring it to a boil. Add the potatoes, carrots and parsnips (leave the beetroot out) and parboil for 10 minutes.
Once they have been parboiled, drain the water, transfer the veggies on a big enough baking tray and add the beetroot, fresh rosemary, garlic head, cutting the upperr part of it, and a good bit of olive oil making sure the veggies are well coated.
Leave your root vegetables roasting for an hour (more if needed). Make sure to stir them occasionally so they cook evenly. Once cooked, take the tray out of the oven and season straight away with sea salt.