Melt the butter in a skillet over medium heat. Add in the onions, season with salt and thyme and leave to cook for approximately 30-45 minutes, stirring occasionally until the onions are soft and caramelized. Add a splash of water if they begins to look dry or burnt.

Preheat the oven to 350 degrees fahrenheit

In a bowl stir together the horseradish and mayonnaise and set aside.

Carefully slice through the buns horizontally so that the bottoms are left connected in one piece and the tops are all connected as one sheet. Place the bottom halves on a baking sheet and then top the bottom buns with the roast beef, the provolone cheese and then all of the caramelized onions. Spread the horseradish mayo over the top half of the buns and then place the top half of the buns onto the sandwiches to close.

In a bowl stir together the melted butter, garlic powder and chopped parsley and brush the mixture all over the top of the buns. Cover the buns with tin foil and transfer to the oven to bake for 20 minutes until the cheese is melting.Remove the tin foil and bake for an additional 5 minutes until the buns are golden brown on top.

While the buns are baking make the au jus. Melt the butter in a sauce pan on medium high heat. Add in the minced garlic and cook for 1 minute before whisking in the flour to form a roux. Whisk in the beef broth and worcestershire sauce until its all incorporated. Leave to simmer for 4-5 minutes until it begins to thicken and then remove from the heat. Season with salt and pepper as needed.

Serve the sliders with a bowl of the au jus for dipping.