Preheat a skillet or griddle over medium heat.
Spread the softened butter evenly on one side of each bread slice.
Toast the bread slices, buttered side down, in the skillet until golden brown and crispy, about 2-3 minutes per side. Remove and set aside.
In a small bowl, mix the mayonnaise and Dijon mustard until combined.
Spread the mayonnaise-mustard mixture evenly on the non-toasted side of three bread slices.
On the first slice with spread, layer 2 leaves of romaine lettuce, 2 slices of tomato, and half of the sliced roast beef. Season lightly with salt and black pepper.
Top with a second slice of bread, spread side up, then spread a thin layer of mayonnaise-mustard mixture on top of this bread slice.
Add the remaining lettuce leaves, tomato slices, and roast beef on top.
Finish by placing the last slice of bread on top, toasted side up.
Press the sandwich gently to compact the layers.
Using a sharp knife, cut the sandwich diagonally into four triangular quarters.
Serve immediately with your choice of side.