Toss all the vegetables, except the tomatoes, in a large bowl with the oil. Season with a little salt and pepper and add the thyme. Tip into a large roasting tin and place in a preheated oven, 190°C (375°F), Gas Mark 5, for 40–45 minutes or until the vegetables are tender. Add the tomatoes and return to the oven for a further 5 minutes or until the tomatoes are just softened and beginning to burst.
Meanwhile, tip the hazelnuts into a small roasting tin and place in the oven for about 10–12 minutes or until golden and the skin is peeling away. Leave to cool, then remove the excess skin and crush lightly.
Toss the rocket leaves gently with the mixed, roasted vegetables and heap on to large plates. Scatter over the crushed hazelnuts and drizzle with the vinegar. Scatter over the mustard cress, if using, and serve immediately,