Perfectly tender, slow cooker roast beef, served over crusty white bread with sandwich toppings piled high!
Pat the roast dry with a paper towel and sprinkle with seasoning and 2 tablespoons of flour. Rub all over to evenly adhere.
Add the oil to a very large, rimmed sauté pan or Dutch oven over medium-high heat and allow to come to temperature. Add the roast to the pan and sear for about 4 minutes on both sides, or until it develops a nice crust. Remove from the pan and set aside on a plate.
Reduce the heat to medium, add the diced onion to the pan and sauté, stirring often. Cook the onion for 3-4 minutes, or until just softened. Sprinkle the remaining 2 tablespoons of flour over the onions and stir to coat.
Add about half of the beef broth one big splash at a time, stirring until completely incorporated after each addition. Once half of the broth has been incorporated, add the salt and pepper and allow to come to a simmer. Simmer for a few minutes, or until thickened slightly.
Add the beef, along with all of its juices, to your slow cooker. Pour over the sauce, along with the remaining broth. Set your slow cooker on the high heat setting and cover. Cook for 4 hours, then remove the beef and shred it. Add the shredded beef back to the slow cooker, and continue to cook (uncovered) for 1 1/2 hours more, stirring occasionally.
Cut your bread for sandwiches (if necessary). If you'd like to toast your bread, preheat the oven to 400°. Once the oven comes to temperature, place the bread on a baking sheet and bake for 4 minutes or until lightly toasted.
Remove bread from the oven, slather with mayonnaise (if using), add a generous serving of shredded roast beef and desired toppings. Serve and enjoy!