Dice the eggplant into roughly ¾in/2cm dice and cut any larger cauliflower florets to give you roughly the same sized pieces. Toss both in the oil, salt and pepper in a large roasting dish so you have all in one layer, as far as possible.
Roast the vegetables for approx 20 minutes until tender.
You can cool the vegetables and store in the refrigerator for a day or two. If using immediately, while they are roasting chop the herbs and blend with the lemon juice, oil and mustard. Pour over the roast vegetables and serve - warm, room temp and cold all work.