Place diced bacon into the instant pot and turn on sauté mode. Cook bacon until crispy. If it starts to stick use a little beef broth to deglaze. Add in white onion and stir well. Add in a little beef broth and deglaze. 

Add in stew beef and garlic. Cook until lightly brown on the outside of the meat. Turn off sauté mode. Add in carrots and diced and peeled potatoes. Pour in one cup of Guinness and the remaining beef broth. If this does not cover your vegetables and meat then add water to just barely cover the vegetables and meat. Add in thyme and bay leaves. Add in salt and pepper. Stir well. 

Place the lid onto the instant pot. Set the valve to the sealed position. Press the pressure cook button or manual and set timer to 25 minutes. Once the instant pot stops counting release pressure. (no this will not make the meat tough by quick release' it will be super tender)

Turn off instant pot and turn on sauté mode. Add in cornstarch slurry (this is 3 tablespoons cornstarch whisked well with 3 tablespoons or so of water). Stir well and as it comes to a boil it will start to thicken the sauce around the stew. If it's still not thick enough you can add a little more slurry. Remember once you turn off sauté mode and let it cool some it will thicken even more just like a gravy.  

Serve over mashed potatoes and topped with fresh thyme.