In a large skillet or dutch oven, heat a couple tablespoons of a high-smoke point oil (canola, vegetable, or grapeseed) over medium high-heat until hot. Add in first batch of cubed beef chuck to pan, be sure not to overcrowd and there's a little space between each piece. Sear on first side until a nice brown crust has formed before flipping. Repeat in batches until everything is cooked. Set beef aside.
In the same skillet or dutch oven, heat a little more oil and add in sliced onions and minced garlic. Sear for a few minutes until onions start browning and softening.
Deglaze the pan by adding in the bottle of Guinness, scraping the bottom with a wooden spoon. Add in beef stock and add the beef back in. Bring to a low boil, cover, and simmer for several hours until the meat falls apart with a fork.
You want the liquid to reduce and become almost like a glaze, but if the beef isn't tender yet and you feel like there's not enough liquid in the pot, add more beef stock and continue reducing down.
For serving, I love puréed parsnip which is the pictures above but mashed potatoes or mashed cauliflower would be amazing! Top with chopped parsley!