Preheat oven to 375 if using baguette. Slice baguette into 1" slices. Toast them in the oven on a baking sheet for about 10 minutes to crisp them up. Remove baguette slices and set aside. If you are using flatbread skip this step.

Set oven to broil. Position top rack on second level from broiler element.

Brush baguette slices or flat bread with blue cheese. You can also use the back of a spoon to spread it.

Chop tomato finely. I set mine in a sieve while I am distributing the cheese so excess juice drains away. (It doesn't take long). While tomato is draining distribute mozzarella evenly over bread. Top with diced tomato.

Broil about 3 minutes just until mozzarella is melted and bubbly. Watch carefully so it doesn't burn.