Instructions

1. Heat the oven to 350°F.
2. Melt 2 tablespoons of the unsalted butter in a small Dutch oven or medium heavy-bottomed pot over medium heat. Add 1 pound thinly sliced large yellow onions, 1/2 teaspoon kosher salt, and 1/2 teaspoon black pepper. Stir well to combine. Cover and cook undisturbed for 5 minutes. Uncover, stir, cover again, and cook for 5 minutes more.
3. Uncover and continue to cook, stirring occasionally at first and then more frequently once the onions start to brown, until the onions are a rich caramel brown color, 15 to 30 minutes. Reduce the heat as needed if the bottom of the pot is getting too dark. Make sure to scrape the bottom of the pot when stirring. Meanwhile, prepare the sandwich.
4. Without separating the rolls, cut 1 (12-ounce) package Hawaiian sweet dinner rolls in half horizontally with a serrated knife. Place the bottom half of the rolls cut-side up in a 9x13-inch or 7x11-inch baking dish.
5. Stir 2 tablespoons mayonnaise, 1 tablespoon Dijon mustard, and 2 finely grated garlic cloves together in a small bowl. Spread all over the cut sides of the top and bottom rolls.
6. Arrange 6 slices of the cheese on the bottom half of the rolls, overlapping the slices as needed to completely cover. Drape and arrange 12 thin slices roast beef in little piles over the cheese, arranging a slice on each roll. Layer the remaining 6 cheese slices over the roast beef.
7. When the onions are ready, turn off the heat. Stir in 2 tablespoons of the water, 2 teaspoons of the Worcestershire sauce, and 2 teaspoons packed brown sugar, scraping up the browned bits from the bottom of the pot. Spread the onions evenly over the cheese; reserve the pot. Top with the top half of the rolls cut-side down.
8. Melt the remaining 2 tablespoons unsalted butter. Stir half of 1 (1-ounce) packet onion soup mix (about 1 1/2 tablespoons) into the melted butter. Brush onto the tops of the rolls.
9. Cover the baking dish tightly with aluminum foil. Bake until the sandwiches are heated through and the cheese starts to melt, about 10 minutes. Uncover and bake until the rolls are lightly browned and the cheese is completely melted, 8 to 10 minutes.
10. Meanwhile, whisk the remaining onion soup mix, 1 teaspoon Worcestershire sauce, and 2 cups water together in the reserved pot (no need to clean). Bring to a boil. Reduce the heat as needed to maintain a simmer and cook until reduced by about a quarter and the onions in the soup mix are tender, about 10 minutes.
11. Transfer the slab of sandwiches to a cutting board. Cut into individual sliders with a serrated knife. Garnish with finely chopped fresh parsley leaves if desired. Serve with the onion jus for dipping.