Rub beef. In a small bowl, mix 1 teaspoon salt, 1 teaspoon pepper, onion powder, and garlic powder. Rub the mixture onto all sides of the beef roast and place it in a 6-8 quart crockpot.
Make gravy. In a small bowl, whisk together the water and Au Jus Gravy mix.
Slow cook. Pour the gravy over the roast and cook on high heat for 5 hours or on low for 8-10 hours.
Make mayonnaise mixture. While the roast cooks, mix mayonnaise, minced garlic, ¼ teaspoon salt, and ⅛ teaspoon pepper in a small bowl. Cover with plastic wrap and refrigerate until serving time.
Caramelize onions. About an hour before the roast is done cooking, melt the butter over medium-low heat in a heavy pan or Dutch oven and add the onions. Cook the onions for 45 minutes on medium-low heat, stirring periodically to prevent burning. The onions will slowly brown, caramelize, and reduce in size. Turn off the heat and place in a serving bowl. Refrigerate if needed before the roast is done cooking.
Shred meat. Carefully remove the roast from the slow cooker and place it on a cutting board. It should be very tender and may break into chunks when removing it from the slow cooker. Using two forks, shred the beef into bite-size pieces. Remove and discard any fatty pieces. Return the shredded beef to the crockpot and serve immediately or keep warm until time to serve.
Serve. To serve, using a slotted spoon or tongs, remove the desired amount of meat and place it on a slider bun. Top with a tablespoon or two of caramelized onions and spread the garlic mayonnaise on the underside of the top bun. Place the top bun over the beef and onions and serve immediately before the juices from the beef cause the slider bun to get too soggy.