Preheat the oven to 350 degrees F. Place the oil in a large skillet over medium heat. Add the onion and bell pepper and cook until tender, about 5 to 6 minutes. While the onion and pepper cook, split the buns and set on a baking sheet. Set two halves of Swiss cheese on the cut sides of each bun. Put the buns in the oven to warm and melt the cheese. When the onions and peppers are tender, add the beef and stock to the skillet and stir until the beef is heated through. Season with salt and pepper. Divide and pile this mixture into the buns.