After defeating Savolla or the frightener, you'll go to the laboratory with Leo. The poor fellow will get himself killed. After a scene Vesemir will tell you to meditate.
While meditating, you can spend the talents you acquired to level up your character, as well as prepare potions. You'll get more information on these subjects in appropriate sections of the guide. Do what you will and click the hourglass to confirm your selection.
Hint 18: You can only meditate in safe spots, by the fire. You can light fire when outdoor by using a flint or an Igni sign. Remember that you can only use sign in locations marked as dangerous.
After meditating look through the locker (M3.6) and take three Swallows from it. Talk to Vesemir to know that the witchers are about to leave Kaer Morhen. Go back upstairs (M3.5).
You'll meet Lambert by the entrance to the kitchen (M2.7). He'll tell you the ingredients you need to prepare a healing potion for Triss. After the conversation he will open the door to the kitchen.
Hint 19: Dialogue options that you've already used are marked by grey color. If a dialogue option opens up a whole new tree, you have to use all of its branches in order to make it turn grey.
Loot the kitchen (M2.8). Go up the stairs (M2.9). There will be two remaining bandits for you to kill.