2) Put the egg yolks into a bowl with the mustard, salt and the white wine vinegar and add the crushed garlic. Put the oil into a measuring cup. Take a whisk in one hand and the oil in the other and drip the oil onto the egg yolks, drop by drop, whisking at the same time. Within a minute you will notice that the mixture is beginning to thicken. When this happens you can add the oil a little faster, but don't get too cheeky or it will suddenly curdle because the egg yolks can only absorb the oil at a certain pace. Taste and add a little more seasoning and vinegar, if necessary.
3) If the mayonnaise curdles it will suddenly become quite thin, and if left sitting the oil will start to float to the top of the sauce. If this happens you can quite easily rectify the situation by putting another egg yolk or 1 to 2 tbsp plus 1 to 2 tsp of boiling water into a clean bowl. Then whisk in the curdled mayonnaise, a half tsp at a time, until it emulsifies again.
4) Chop the membrillo and warm gently in a little saucepan until it melts. Cool and add to the mayonnaise with the rosemary and oil. Taste and correct the seasoning if necessary.
For the roast rack of lamb:
1) Preheat the oven to 220C/Gas 7. Score the skin of the lamb – you may like to insert a few tiny sprigs of rosemary and slivers of garlic here and there on the skin side. Season with salt and freshly ground pepper.
2) Roast for 25 to 30 mins, depending on the age of the lamb and the degree of doneness required.
3) Allow to rest for 5 to 10 mins. Carve, allowing 2 to 3 cutlets per person, depending on size. Garnish with a sprig of rosemary and serve with the rosemary and membrillo aioli.
Cook's Note: *Membrillo is available from delis and many good cheese shops.