Preheat oven to 350°F. Take the rolls out of the package, and using a large knife, slice the rolls across horizontally, separating the tops and the bottoms. Place the bottom, cut side up into an 11x7 or 13x9 baking dish.

Melt butter in a large skillet over medium heat. Add onions and cook, stirring occasionally for 12-15 minutes, or until onions are browned and softened.

Add thyme, salt, and pepper, and stir to combine. Remove and set aside.

Top the bottom of the rolls with 6 slices of cheese. Add the roast beef, and top with the onions, using a spatula or tongs to spread them out. Add the remaining cheese, and lastly add the tops of the rolls.

In a small bowl stir together melted butter and onion soup mix. Brush the butter mixture on top of the rolls.

Cover loosely with foil and bake in preheated oven for 25 minutes.

Remove foil and bake for an additional 5 minutes, or until the tops are browned.

Prepare the au jus according to the package directions.

Garnish the French dip sliders with fresh chopped parsley and serve with au jus on the side for dipping.