1. To make the dressing, whisk together the tahini, garlic, yogurt and oil. Whisk in the lemon juice until smooth, then add 3 tbsp water and season. You might need to add a little more water – the mixture should be slightly thicker than double cream, but only slightly. Cover and chill until needed (take it out of the fridge 30 minutes before serving or it will be too firm).
2. Wash the squid and pat dry thoroughly on kitchen paper (if they aren’t dry they won’t cook well). If they’re small, leave them whole. If they’re larger, cut off the tentacles (if still attached) and set aside. Cut the bodies down one side to open them out, then halve lengthways and score on the inside with a criss-cross pattern, without cutting all the way through.
3. Put the squid (and tentacles, if separated) in a bowl with the oil. Heat a griddle pan until really hot. Season the squid and cook in batches, pressing down to pick up the griddle marks. They only need about 30 seconds on each side to become golden and cooked. As each batch is ready, transfer to a large bowl and squeeze over the lemon juice.
4. Once all the squid is cooked, add the chillies to the pan and cook briefly. Tip into the bowl with the squid, add the dill and spinach and toss together. (The spinach leaves will wilt in the residual heat.) Put everything on a big platter and serve the tahini sauce on the side, with a few more lemon wedges, if liked.