Preheat oven to 350 F and line a baking sheet with parchment paper.

In a large skillet over medium heat, melt the butter and add the sliced onions. Cook, stirring often, for about 15 minutes until the onions are soft and caramelized. Remove from the heat and then season with thyme, salt, and pepper.

Use a bread knife to slice the buns in half horizontally across then place the bottom buns onto the prepared baking sheet.

Add the roast beef slices in an even layer, top with a layer of Swiss cheese, add the caramelized onions, and then place the top of the buns back on.

In a small bowl, place the butter slices and dijon mustard. Microwave for 10 seconds, just until melted. Stir to combine.

Brush the butter mixture evenly on top of the buns and then sprinkle on the dried onion soup mix.

Tent with foil and bake for 15-20 minutes until the cheese is melted. Remove the foil and then bake for an additional 5 minutes if needed to toast the buns.