Preheat the grill to medium heat. Allow the cast iron to preheat as well.
Add a little oil and the tomatoes to the pan and allow them to brown, blister and cook until they just begin to rupture and naturally macerate.
Move the pan to indirect heat and add the mozzarella. Allow it to cook indirectly until the cheese is completely melted. If you have a lot of excess moisture, carefully (using oven mitts) pour off the excess liquid into a bowl to be discarded.
Remove from the heat and garnish with salt, pepper, basil, and balsamic (optional).
For the bagel chips - Slice the bagel into small, 2-3" crostini rounds and brush both sides lightly with oil. Grill directly until each side is lightly golden brown. Serve with the dip!