Mix the olive oil, lemon zest, lemon juice, garlic powder and celery salt with the herbs and crushed garlic in a bowl and set aside.
Put the prepared Purple Majesty potatoes, onions, leeks, peppers, parsnips and carrots in a large, deep roasting tin. Now drizzle all over with the lemon and herb marinade and season with freshly ground black pepper.
Roast for 25 minutes, stir everything around, then add the cherry tomatoes and stir again so that everything browns evenly.
Roast for a further 25-30 minutes, or until cooked through and nicely browned on the edges.