To make the pickled onions, put the sliced onions in a bowl and add the vinegar and sugar with a little pinch of salt. Toss together then leave to macerate, stirring now and then. The onions will eventually become really soft and turn a lovely bright pink.
Mix the tahini, yogurt, dill and halved garlic clove. (You’ll be scooping the garlic out later, the idea is just to give a really subtle hit to the sauce). Add enough water to get a drizzling consistency and chill until needed.
Heat the oven to 200C/fan 180C/. Put the butter in a small pan with the spices and a pinch of salt. Melt together, then cook very gently for 1-2 minutes. Put the caulis in a roasting tin or on a baking sheet. Brush over the butter then turn over to coat in as much of the butter as possible (but don’t worry if you don’t hit every spot).
Cover the whole tray with foil and cook for 20 minutes, then take the foil off and cook for another 20-30 minutes, or until tender and starting to catch a little at the edges.
Scoop the garlic out of the sauce and drain the pickled onions.
You can serve this from the tray or transfer the caulis to a platter, or individual plates. Scatter with the onions then drizzle all over with the sauce. Serve with a bit of extra dill, enjoy!