In a small bowl, combine the softened butter, miso paste, and lemon juice. Mix until smooth and well combined.
Spread a generous layer of the miso butter mixture on one side of each slice of bread.
Heat a skillet over medium heat. Place two slices of bread, buttered side down, in the skillet.
Layer the tomato slices evenly on top of the bread in the skillet. Season with salt and pepper.
Top with the remaining slices of bread, buttered side up. Cook for about 3-4 minutes, or until the bottom is golden brown.
Carefully flip the sandwiches and cook for another 3-4 minutes, until the other side is golden and the tomatoes are slightly softened.
Remove from the skillet and let cool for a minute. If desired, add fresh basil leaves before slicing in half and serving.