Pat dry the beef cubes with a paper towel. Make sure the beef is at room temperature versus shocking it with heat if it's cold from the fridge.Liberally salt and pepper all sides of the beef cubes. Over medium-high heat, coat a dutch oven or an Instant Pot with olive oil, around 2-3 tablespoon.
In batches, sear all sides of the beef cubes. Let the beef rest on a plate while chopping the other ingredients. In the same pot, add another tablespoon of olive oil, lower the heat, and add the carrots, garlic, leeks, shallots, and a pinch of salt, and some cracked pepper. Let it cook for 3 to 4 minutes.
Add the tomato paste, and stir. Add the two cans of Guinness, and add the thyme leaves and bay leaves. Stir and let it come to a simmer. Slowly add the beef cubes along with their juices into the pot. Stir, cover, and let it cook over low heat for 2 ½ hours on the stovetop.
30 minutes before the stew is done, peel and cut the potatoes and boil until fork-tender, 8-10 minutes. Drain, and return the potatoes to the same cooking pot to dry out (no heat!). Add increments of ½ cup of milk along with smashed garlic and butter.
Whip with hand mixer, adding a little milk at a time until it's smooth and whipped. Add salt and pepper to your liking. Optional, add some parmesan cheese!
Add some chopped fresh parsley to the stew, check for season.Dollop some mashed potatoes, along with Guinness Beef Stew. The meat will be super tender, and will fall apart in your mouth. There's no shame to eat this stew with a tall glass of Guinness!