Heat the oil in a non-stick pan over medium heat. Once hot, add in the mushrooms and onions; sauté until they become soft and golden brown (about 5 to 7 minutes). Add in the roast beef and sauté until browned. Season with salt and pepper.
Layer the bread with cheese, beef, onions, mushrooms, another slice of cheese and bread slice.
Melt the butter in a non-stick pan over medium-low heat and place the sandwich in the pan.
Cover with a lid (watch the heat to make sure the bread isn't burning). When the cheese is melted and the bread is nicely grilled to a golden brown, turn the sandwich over and repeat with the other side.