Heat your oven to 220°C/Fan 200°C/Gas 7. Set a large roasting tin on the top shelf to warm up. Pop the sundried tomatoes in a bowl. Cover with water and set aside to soak.
Cut the cauliflower into medium florets. Discard larger leaves and keep the small ones. Add the florets and leaves to the warm dish. Season well. Add 1 tbsp oil. Roast for 25 mins or till the edges caramelise.
Halve, peel and thinly slice the onion. Add it to the cooked cauli and put back into the oven for 5 mins.
Slice 1cm off the top and bottom of the oranges. Carve the peel from the sides. Cut the segments out of the white casings. Tumble into a bowl. Squeeze any excess juice from the peel scraps and pith into the bowl.
Drain and thinly slice the sundried tomatoes. Add to the orange with the olives. Add to the cooked cauliflower and onion. Pile into a serving dish. Strip off the oregano leaves. Scatter over the top, along with 1/2 tbsp olive oil and a pinch of pepper.