First, cut each chicken liver in half then trim off any sinew, blood vessels or discoloured bits.
Place the chicken livers into a bowl, give them a good rinse under cold water then drain well. Transfer the livers to a plate then pat dry with kitchen paper.Rinsing the livers removes any blood, which can make the mousse taste bitter. Set a large frying pan over a medium-high heat then add in the tbsp of veg oil & leave to heat up for a minute or two. Once hot, add in the livers then cook for 2-3 minutes on each side, until well seared but still pink inside (like a medium steak!). Transfer the cooked livers to a plate then set to one side, to cool.
Turn the heat down to medium then add the butter into the pan. Once melted, add in the diced shallots, the cayenne pepper & pinch of salt then cook, stirring regularly until soft & caramelised. This will take 5-10 minutes.
Next, stir the chopped thyme into the shallots then deglaze the pan with the sherry, making sure to scrape the bottom of the pan to release all the brown bits. Keep cooking, until all of the alcohol has been evaporated then transfer the shallots to a bowl & leave to cool.
Once the livers & shallots have cooled, add both into a jug blender then turn on. Blend for a minute, to break the livers up then add the soft butter in a small amount at a time (with the blender running still). Make sure to let each piece of butter incorporate fully before adding in more.If there's any resting juices from the livers, don't add them in as they can affect the texture of the mousse. Add the mascarpone & lemon zest into the blender then continue blending until incorporated. Make sure to scrape the sides of the blender down regularly. Next, gradually pour the double cream into the livers (with the blender running). Once all the cream has been added, continue blending for a couple of minutes, until the mousse is smooth.It's important to add the cream in slowly so that the mousse emulsifies properly. If it looks split, keep blending to bring everything together. Add the lemon juice to the mousse then season well with salt & pepper. Blend again, to combine then give the mousse a taste to check the seasoning, adding more salt & pepper as needed.The mousse can take quite a lot of seasoning! Pass the mousse through a sieve, into a serving dish, cover the surface with a piece of clingfilm (this stops the mousse from oxidising) then refrigerate until set. This will take at least a couple of hours.