Put all the ingredients into a large, clean saucepan and whisk over low heat until the mixture forms a thick froth on the top and starts to come to the boil. Stop whisking immediately and let the mixture rise to the top of the saucepan, taking care not to let it boil over. As soon as it reaches the top, remove the saucepan from the heat and wait for the mixture to subside. Return the pan to the heat and allow the mixture to rise and subside once more in the same way. Repeat the process once more, then set aside for 5 minutes.
Meanwhile, scald a large bowl, a sieve and a large piece of muslin or a clean tea towel with boiling water. Drain both bowl and sieve well and wring out the muslin or tea towel. Line the sieve with the piece of muslin, folded double, or the tea towel, and place it over the bowl. Taking care not to break up the froth, pour the aspic jelly carefully into the sieve and leave undisturbed to strain into the bowl. Do not press or squeeze. Cool the aspic and use immediately. Alternatively, it can be refrigerated for up to 24 hours.
If you do not feel conﬁdent about making your own aspic jelly, you can buy packets of aspic jelly powder from grocers or supermarkets. Simply dissolve the powder in boiling water according to the manufacturer's instructions.