Start your grill (gas or charcoal)
Mix mayonnaise and pesto in a small bowl and refrigerate until used
Brush tomato and onion slices with olive oil, and season with black pepper
Place on hot grill until charred, 1 to 3 minutes
Turn and season with additional pepper and char the second side
Place the charred veggies on a plate
Lightly brush some olive oil on the baguette, then place on the grill.
Get a good toast on the bread, 30 to 45 seconds depending on the heat of the grill. Carefully hold the bread down if necessary to get an even toast,then remove from the grill.
Spread the cut surface of the toasted baguette with the mayo/pesto sauce.
Assemble the sandwich with torn mozzarella pieces, grilled tomato and grilled onion.
Top with torn fresh basil and flat-leaf parsley
Finish with a drizzle of balsamic vinegar