Heat the olive oil in a large skillet over medium-high heat. Add the beef chunks and brown on all sides, about 5 minutes total. Transfer the beef to the slow cooker.
Add the sliced leeks and minced garlic to the same skillet. Cook for 3 minutes until the leeks soften slightly. Scrape everything into the slow cooker.
Pour the Guinness into the slow cooker, followed by the beef broth and tomato paste. Stir to combine. Add the carrots, potatoes, thyme, salt, and pepper.
Cover and cook on low for 7 to 8 hours, until the beef is tender and the vegetables are soft.
In a small bowl, stir together the sour cream and horseradish. Taste and adjust the horseradish amount if you prefer more heat.
Ladle the stew into bowls and top each serving with a generous spoonful of the horseradish cream.