Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Lightly grease a 23x33cm (9x13 inch) baking dish. Slice the sheet of slider buns in half horizontally without separating them, creating a top and bottom slab. Place the bottom slab into your prepared dish.

Caramelise the Onions: In a large frying pan over medium heat, melt the 2 tablespoons of butter. Add the thinly sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for 10-15 minutes until they are soft, deeply golden, and sweet. What works best for me is keeping the heat on medium-low to prevent them from burning before they have a chance to sweeten.

Create the Jus: Once the onions are caramelised, pour in the beef stock and Worcestershire sauce. Use a wooden spoon to gently scrape any browned bits from the bottom of the pan—this is where so much flavour is hiding! Add the dried thyme, bring to a simmer, and let it cook for 5-7 minutes until it has reduced slightly. Season with salt and pepper to taste.

Assemble the Sliders: Using a slotted spoon, remove about half of the onions from the jus and spread them evenly over the bottom slab of buns in the baking dish. Layer the thinly sliced roast beef over the onions, followed by the slices of Swiss cheese. Place the top slab of buns on top.

Make the Butter Topping: In a small bowl, whisk together the 85g of melted butter, minced garlic, chopped parsley, and onion powder.

Brush and Bake: Pour or brush the garlic butter mixture evenly over the top of the sliders, ensuring it seeps into all the crevices. Sprinkle with poppy seeds, if using. Cover the dish with foil.

Bake to Perfection: Bake in the preheated oven for 15 minutes. Then, remove the foil and bake for another 5-10 minutes, or until the tops are golden brown and the cheese is completely melted and bubbling at the edges.

Rest and Serve: While the sliders are baking, gently re-warm the remaining jus in the pan. Once the sliders are done, let them rest in the dish for about 5 minutes before slicing into individual portions. This helps them hold together. Serve immediately with the warm jus on the side for dipping.