Prepare Your Ingredients: Start by gathering all the ingredients. If you’re using onions, slice them thinly and set them aside.

Heat a Skillet: Preheat a large skillet over medium heat. Add a bit of butter to the pan, allowing it to melt and coat the bottom.

Cook the Onions: If you’re adding onions, toss them into the skillet and sauté until they are soft and caramelized, about 5-7 minutes. Remove them from the skillet and set aside.

Assemble the Sandwich: Take two slices of bread and spread a generous layer of butter on one side of each. On the unbuttered side, layer the roast beef, followed by the sautéed onions, and top with a thick slice of cheddar cheese.

Grill the Sandwich: Place the sandwich in the skillet, buttered side down. Grill for about 3-4 minutes until the bottom is golden and crispy. Press down gently with a spatula to ensure even cooking.

Flip and Finish: Carefully flip the sandwich and grill the other side for another 3-4 minutes, until the cheese is melted and the bread is toasted to perfection.

Serve: Once both sides are golden brown and the cheese has melted, remove the sandwich from the skillet. Slice it in half and serve hot with a side of pickles or chips!