In a bowl mix together the sour cream and horseradish. Season with salt and pepper to taste.
In a nonstick pan on medium low heat. Butter one side of the bread slices with butter. Spread the sour cream horseradish onto the bread with cheese on top of each bread slice. Place tomatoes seasoned with salt and pepper on one slice of bread and roast beef on the other. Place the bread butter side down in the pan.
Cover with a lid. (Watch the heat, make sure the bread isnt burning) When the cheese is melted and the bread is nicely grilled to golden brown, put the two slices of bread together and then remove from heat.