1.5 kgcarrots, coarsely chopped (about 12)2onions, coarsely chopped1garlic head, halved horizontally60 ml (¼ cup)olive oilFinelygrated rind and juice of 3 lemons and 1 orange600 mlbuttermilk250 gmthick plain yoghurt3 tbspfinely chopped dill, plus extra sprigs to serve2spring onions, thinly slicedTo serve:pea shoots (optional)
Preheat oven to 200C. Scatter carrot, onion and garlic on oven trays lined with baking paper, drizzle with oil, scatter with lemon and orange rinds, cover with foil and cook until just tender (1-1½ hours). Remove foil, roast until golden and very tender (45 minutes-1 hour), then cool to room temperature.
Squeeze garlic from skins and return to carrot mixture. Process carrot mixture in a food processor in batches, adding buttermilk, orange juice, two-thirds of the lemon juice and 600ml iced water, until smooth. Pass through a coarse sieve, pressing to extract all liquid (discard solids), season to taste, refrigerate until chilled (3-4 hours).
Meanwhile, combine yoghurt, dill, spring onion and remaining lemon juice in a bowl, season to taste. Serve soup with yoghurt, scattered with pea shoots and extra dill sprigs.