In a large bowl, mix up the eggplant, zucchini and portobello mushroom slices.
Drizzle the veggie mixture with some extra virgin olive oil, and season with some ground black pepper and salt.
Now, heat a medium non-sticking grill pan over the stove on medium-high heat.
Once warm, place the veggie mixture in the grill pan and sear the veggies for few minutes on each side, or until they soften and develop some grill marks.
Remove the grill pan from the heat and place the grilled veggie mixture on a large serving plate.
Sprinkle it with some balsamic vinegar and set it aside to allow it to cool and settle.
In the meantime, spread 1 tablespoon of pesto on the bottom piece of the sour-dough bread and set one piece of mozzarella on top.
Now, carefully layer the grilled veggies on top of the mozzarella.
Top the veggies with more cheese slices.
Put the top piece of sour-dough bread on and lightly brush the external side with the remaining extra virgin olive oil.
Place the sour-dough bread back on the pre-heat grill pan and sear until the cheese slices are completely melted and the external side is nice and crunchy.
Your grilled veggie panini is now ready; place it on a plate, and garnish if you like. Serve warm and enjoy!