Season the beef chunks with 1 1/2 teaspoons kosher salt and 1 teaspoon black pepper. Heat the vegetable oil in a large skillet over medium-high heat until shimmering. Working in batches if necessary to prevent overcrowding, add the beef in a single layer and sear for 1 to 2 minutes per side, until nicely browned all over. Once browned, turn off the heat and transfer the beef to the slow cooker.

Pour the beer and beef broth into the hot skillet and stir, scraping up any browned bits stuck to the bottom—these add rich flavor to the stew. Let the mixture sit in the pan briefly while preparing the remaining ingredients.

To the slow cooker, add the carrots, parsnips, potatoes, onion, garlic, minced sage, beef base, and tomato paste. Pour the beer and broth mixture from the skillet into the slow cooker over the beef and vegetables. Cover with the lid and cook on LOW for 7 to 7 1/2 hours, until the beef and vegetables are tender.

In a small bowl, mix the softened butter and flour together to form a paste. After the initial cooking time, remove the lid and stir in the prepared horseradish along with the butter-flour mixture. Stir well to incorporate, then re-cover and continue cooking for another 20 to 30 minutes, stirring occasionally, until the stew thickens slightly.

Taste and adjust seasoning with additional salt and pepper if needed. Garnish with freshly chopped parsley before serving.