In a bowl, mix together the sour cream and horseradish. Season with salt and pepper to taste.
Spread the sour cream-horseradish paste onto both slices of bread. Layer the bread with cheese, spinach, roast beef seasoned with salt and pepper, tomatoes, another slice of cheese, and remaining slice of bread.
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.