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, shaking off excess flour and add to the pan in a single layer, working in batches if needed. Brown for 2-3 minutes per side and then transfer the beef to the slow cooker.
Add the additional tablespoon of oil to the stockpan, along with the minced onion. Cook, stirring often for 3-5 minutes, or until the onion has softened and caramelized slightly. Add garlic and cook for 1 minute.
Add ½ cup dry red wine (or additional beef stock) to the pan and scrape up any browned bits off the bottom of the pan. Let this mixture simmer for just a minute or two to reduce and then remove from the heat. Whisk in the 6 ounces tomato paste to incorporate and then add this mixture to the slow cooker.
Add the 4 cups low-sodium beef stock, 2 tablespoons Worcestershire sauce, 1 tablespoon prepared horseradish, 1 teaspoon dried thyme leaves, 1 dried bay leaf 1 teaspoon each salt and pepper, potatoes and carrots into the slow cooker.
Cover and cook on low for 8-10 hours or high for 3 to 4 hours.
To thicken, whisk 2 tablespoons cornstarch with 3 tablespoons 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 2 tablespoons unsalted butterand stir until melted.
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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 to 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 or 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.