Place the beef in a large storage bag, add 1 teaspoon kosher salt, 1 teaspoon black pepper, and flour. Close and toss to coat.
Heat 2 tablespoons of olive oil in a large, deep skillet or stock pan over medium-high heat for 1-2 minutes. Once the oil has heated, remove the beef from the bag, shake off excess flour, then add it to the pan in a single layer, working in batches if needed. Brown for 2-3 minutes per side, then transfer the beef to the slow cooker.
Add the remaining 1 tablespoon of oil to the stockpot used to brown the beef. Add the minced onion and cook, stirring often, for 3-5 minutes, or until the onion softens and slightly caramelizes. Add garlic and cook for 1 minute.
Add the dry red wine (or additional beef stock) to the pan and scrape up any browned bits from the bottom. Let this mixture simmer for just a minute or two to reduce, then whisk in the tomato paste to incorporate and continue cooking for 1 minute. Remove from the heat, then add this mixture to the slow cooker.
To the slow cooker, add beef stock, Worcestershire sauce, prepared horseradish, dried thyme leaves, bay leaf, potatoes, carrots, and 1 teaspoon each of salt and pepper, then stir to combine.
Cover and cook on low for 8-10 hours or on high for 3-4 hours.
To thicken, whisk the cornstarch with water in a small bowl until incorporated. Slowly add it to the stew, stirring to incorporate. Cover and let cook on high for 10 minutes.
Uncover, turn off the heat, and add the butter and stir until melted. Serve as desired.
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Notes
Beef: A chuck roast is always best for beef stew, but a brisket, shank, or package labeled stew meat works as well.Red Wine: A dry red wine that is suitable for drinking is best. Select a merlot, malbec, or cabernet OR use additional beef broth in place of wine.Horseradish: Be sure to use prepared horseradish, not horseradish sauce. (The ingredients are just grated horseradish, salt, and sometimes vinegar.) Potatoes: Use red, new, gold, or russet potatoes. Keep in mind russet potatoes have a high starch content, so they will not hold their shape as well as gold or red potatoes.
Storage: Leftovers can be stored in the refrigerator in an airtight container for up to 4 days. You can opt to freeze the stew in a freezer-safe container for up to 3 months, but the texture of the potatoes may be altered after freezing.