Fill a medium saucepan with cold water. Add the carrot, celery, bay leaves, juniper berries and season with salt and pepper. Lastly add the octopus. Put the saucepan on the fire, bring slowly to the boil, then simmer for about an hour. Let the octopus cool in the water before lifting it out.
Cut the octopus into 4 to 5 pieces and press down into a empty plastic water bottle, with the top part cut off so that the octopus can be pressed into it. Prick the bottom of the bottle so that all the liquid will drain out. With the scissors cut the empty top part of the bottle downwards to form upright strips and press them down towards the centre to form a lid.
Roll the bottle tightly with cling film. Place the bottled octopus in the fridge for at least 24 hours.
Remove the cling film from around the bottle and cut along one-side of the bottle with the scissors and press out the octopus roll. Slice finely with a very sharp knife and serve with the lemon sauce made by mixing all the ingredients together. Decorate with cherry tomatoes and rucola.