For the purée, remove any leaves from the cauliflower and discard. Chop cauliflower into small pieces.
Heat the olive oil and the butter in a medium saucepan until the butter foams, then add the onion, garlic and a little sea salt. Stir over low heat until onion is soft but not coloured, about 10 minutes.
Add cauliflower pieces and chicken stock and simmer gently for about 10 minutes, or until the cauliflower is tender and most of the liquid has been absorbed (be careful not to catch the cauliflower on the base of the pan).
Drain any excess liquid, then transfer mixture to a food processor. Process until very smooth. Then, with the motor still operating, very slowly add the lemon juice and oil in a thin stream until combined. This will lighten the purée. Season with sea salt and pepper and return to the saucepan. Keep warm on a very low heat while preparing the scallops.
Pat scallops dry with paper towel. Season with salt and pepper. Heat butter in a large frying pan on a high heat. When the butter starts to foam, stir constantly until the colour turns light brown. Add the scallops and pan fry for 1 or 2 minutes on each side or until golden, but not cooked all the way through. Add capers and remove from the heat.
Divide the purée between four plates. Place three scallops on each, then drizzle with the burnt butter and caper sauce. Finish with lemon zest, a little sea salt and a good grind of pepper.