All you do is cut the vegetables into large, chunky pieces (no smaller than 1½ inches/4 cm) – leaving the celeriac until last, as it may discolour if left for too long.Place in a large bowl, then add the herbs, garlic, olive oil and lots of seasoning and just use your hands to mix them.The prepared vegetables can now be kept in a sealed plastic bag in the fridge for 2-3 days. When you're ready to cook the vegetables, spread them out on the baking tray and cook in the pre-heated oven on a high shelf for 30-40 minutes, until they're tender and turning brown at the edges.