Melt butter in the pot of a multi-functional electric pressure cooker (such as Instant Pot(R)) set to Saute mode. Add mushrooms and saute for 3 minutes. Add garlic and stir until fragrant. Slowly pour in chicken stock and bring to a boil while scraping any browned bits off the bottom of the pot.
Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 3 minutes. Allow 10 to 15 minutes for pressure to build. Meanwhile, mix cream and flour together in a bowl to form a slurry.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Pour slurry into the soup; stir to combine.
Switch pot to Saute mode. Stir until soup is slightly thickened. Add garlic salt, thyme, and pepper.
