Heat oil and butter together in a skillet until butter has melted. Add mushrooms and onions. Cook until browned. About 15 minutes. Season to taste with salt and pepper.
Cut rolls horizontally and place the bottom half of the rolls in a cast iron skillet or onto a baking sheet.
Layer roast beef and Swiss cheese on the rolls. Top with sautéed mushrooms and onions.
Place the roll tops on top.
In a small bowl whisk together melted butter and thyme. Brush onto rolls.
Bake 15 - 20 minutes or until cheese has melted and sliders are warmed through. Serve immediately.