Hi pietro, welcome to the forum.  I could find no lemon mousse recipe in our books but found this one which would be easily converted for the TMX if you are interested.
http://www.bbcgoodfood.com/recipes/4362/lemon-mousse

I would beat the egg whites first then set aside.  Making sure the TM bowl is perfectly clean and dry, place the butterfly in place, add the eggs and beat for 5 minutes/50^o/speed 3. Remove from bowl and set aside. No need to clean the bowl or the butterfly, add the cream, lemon zest and I'd use icing sugar not castor sugar. Whisk on speed 4 until the cream just starts to thicken. Don't leave it, keep an eye on it - it will depend on how fresh your cream is as to how long it takes to get to this stage but it won't be long.  I watch it through the hole in the lid with a torch. Add the lemon juice and continue beating until it thickens further then fold the beaten egg whites into the mixture by hand with a spatula.