A combination of crispy roasted root vegetables is an essential part of any British roast dinner. I heap them all together on a serving dish and let my guests serve themselves. The vegetables can be prepared and parboiled, then set aside until you’re ready to pop them into the oven to roast.
Preheat your oven to 350 degrees F (180 degrees C/Gas mark 4).
Put the potatoes, sweet potatoes and parsnips into a large pot of cold salted water. Bring to a boil, then turn the heat down to a gentle simmer. Cook for 5 minutes and then drain in a colander. (If you are preparing ahead of time, you can now set the vegetables aside, or keep them in the fridge until you are ready to roast.)
Pour the vegetable oil into a large roasting pan and heat it in the oven for 5 minutes. Add the vegetables to the hot oil and toss to coat. Sprinkle with the salt and pepper and add the rosemary sprigs. Carefully toss again.
Pop the vegetables in the oven and roast for 30 minutes. Turn the vegetables and add the garlic, then roast until the vegetables are crispy and golden on the outside, about 30 minutes more.