Gather all ingredients. Preheat the oven to 350 degrees F (180 degrees C). Grease a 9x13-inch baking dish.
Slice the slider buns in half horizontally without separating them, and place the bottom half in the prepared dish.
In a heavy skillet over medium heat, melt 1 tablespoon butter; add onion and cook, stirring occasionally, until soft and golden, 8 to 10 minutes.
Layer roast beef evenly over bottom buns, then layer cheese slices over beef. Scatter caramelized onion over cheese, and add top buns back on.
Melt remaining butter and stir in garlic powder, Worcestershire sauce, and parsley; brush over tops of buns. Cover with foil.
Bake in the preheated oven for 15 minutes. Uncover and bake until tops are golden and cheese is melted, about 5 minutes more.
Meanwhile, for jus, combine bone broth, Worcestershire, soy sauce, garlic powder, onion powder, and fish sauce in a small saucepan. Bring to a simmer, stirring occasionally, over medium heat. Stir cornstarch and water together in a small bowl to create a smooth slurry; add to the broth, and simmer until slightly thickened, 2 to 3 minutes.
Serve sliders with the jus for dipping.
