Heat the oven to 350°F.
Heat the olive oil and butter in a medium skillet over medium heat. Add the onions and season with salt and pepper. Cook until soft, about 7-8 minutes.
Add the beef stock, Worcestershire sauce, and garlic powder and stir. Reduce the heat to medium low and simmer until most of the liquid is mostly gone, about 10 minutes.
While the onions are simmering, spray a 13 X 9 cooking pan with cooking spray.
Remove the tops of the buns and place the bottoms in the pan.
Note: some buns may need to be cut in half.
Place a slice of roast beef on each bun. Place the slices of cheese side by side until the meat is covered.
Once the onions are ready, pour them over the cheese. Place the bun tops on top.
Mix the melted butter, beef base, garlic powder, and onion powder in a small bowl. Using a basting brush, brush the mix over the bun tops. Sprinkle some poppy seeds over top.
Put the pan in the over for 10 minutes. Use a knife to slice the sliders apart and serve!