Prep Beef: Place 3 pound chuck roast on a large plate. In a small bowl combine 3 teaspoons kosher salt, 2 teaspoons onion powder, 2 teaspoons garlic powder, & ½ teaspoon ground black pepper; stir to combine. Pat chuck roast dry with a clean paper towel and cover in the seasoning ensuring it is fully coated. Set aside.
Slow Cooker Ingredients: Add 4 cups beef broth, 1 packet french onion soup mix, 1 sweet yellow onion (sliced), 1 tablespoon garlic, 2-3 sprigs fresh thyme, 3 teaspoons worcestershire sauce, 1 teaspoon liquid smoke and 1 teaspoon dijon mustard to the slow cooker. Mix and scoop out 1/4 cup of the broth mixture and set next to the stove.
Sear Beef: Bring a large cast iron skillet to medium-high heat. Add 2 tablespoons butter and let melt. Sear chuck roast on all sides. Transfer to slow cooker.
Deglaze the Pan: In that hot skillet that seared the roast, add the 1/4 cup beef broth mixture and scrape up all the bits that are sticking to the pan. Add the remaining 2 tablespoons butter and let melt. Once the bottom of the pan is scraped clean, transfer the liquid to the slow cooker and pour over the top of the chuck roast.
Let it Cook: Cover the slow cooker and cook on HIGH 4 hours or LOW 8 hours. Shred the beef with two forks before serving.
Assemble Sandwiches: Butter the inside of 8 hoagie rolls, add desired amount of onions and shredded beef, add cheese on top (and a little horseradish mayo, if desired).
Serve: Enjoy each sandwich with a small bowl of broth (or au jus) from the slow cooker for dipping.
Notes
Dairy-Free: use a plant-based butter and dairy-free cheese.
Gluten-Free: simply use a gluten-free bun of your choice.
Storage: store leftover cooked beef in an airtight container in the refrigerator for up to 3 days - store broth (or au jus sauce) in a separate container.