Marsala, a fortified wine from Sicily, gives this slow cooker soup recipe its wonderful flavor. Feel free to substitute sherry for a similar effect.
Pour 2 cups boiling water over porcini mushrooms. Let stand 20 minutes. Drain porcini mushrooms in a colander over a bowl, reserving mushroom broth. Strain mushroom broth through a cheesecloth-lined colander into a bowl; discard solids. Add 2 cups water and next 4 ingredients (through pepper) to mushroom broth; set aside.
Remove lid from a 6-quart Instant Pot®. Press [Sauté]; use [Adjust] to select "More" mode. When the word "Hot" appears, swirl in oil. Add shallots and garlic; cook, stirring constantly, 4 to 5 minutes or until shallots are soft. Stir in Marsala. When mixture comes to a boil, cook 30 seconds. Turn cooker off.
Add porcini mushrooms, broth mixture, thyme, and sliced mushrooms. Close and lock the lid of the Instant Pot®. Turn the steam release handle to "Venting" position. Press [Slow Cook], and use [Adjust] to select "More" mode. Press [-] or [+] to choose 4 hours cook time. Ladle soup into 12 bowls. Garnish with parsley, if desired.