Now, of course, you can change the ingredients to fit your taste. You can make it with meat too, such as sliced meats or a sliced turkey or chicken breast. My version is vegetarian and I used what I had in hand (mushrooms, zucchini, mozzarella).
The sky is the limit when it comes to the filling, but the only think to keep in mind is to avoid items that tend to loose liquid because it will make the sandwich soggy. That is why I used fresh mozzarella that is not packed in water, and squeezed all excess oil from the sun-dried-tomatoes.
Cut the top of a sourdough bread and remove the insides of the bread, leaving a small border.
Start layering with all your favourites. For this one I did:
Grilled sliced zucchinis
Mozzarella (not packed in water, or if not possible, squeeze all excess liquid)
Sun-dried-tomatoes (excess oil removed)
Sliced white mushrooms, cooked in olive oil.
Continue layering until the bread is filled. Brush the lid with pesto and close the sandwich. Wrap it in plastic and keep refrigerated until ready to eat.