This page of the Control guide contains a walkthrough for a side mission called Fridge Duty. You will learn, e.g. where to find the room with the fridge, and how to defeat the Former boss.
To begin the quest, travel to level four of Panopticon, right next to the checkpoint. Talk to the person locked in the room with the fridge.
Walkthrough: During your first visit, you won't be able to help the man. Come back after you've visited K6 Cell during My Brother's Keeper main mission. Talk to the man and go to Langston. He will open the door - go back to the chamber, open the door, and it will turn out he was killed by an item of power.
Try to clean the fridge from Hiss - you'll enter the astral plane. You'll now have to face a boss called Former. The enemy is a giant creature that shoots bullets at you and attacks with its big legs. The key to success in combat is avoiding attacks and looking under your feet - the monster is able to destroy the floor, making you easily fall into the abyss, resulting in a rebirth at the checkpoint.
Avoid attacks, hide behind pillars and use telekinesis. Levitation is also useful to keep away from the destroyed floor. Once you defeat the enemy, go to Langston. You'll receive a reward - 3 skill points. In addition, you will automatically start Langston's Runaways - another side mission.
A detailed strategy for the fight with the Former can be found in the Bosses chapter.