Cut the baguette in half horizontally. Spread the pesto evenly on the insides of both sides of the baguette. Fold the mortadella to fit the baguette and shingle it along the bottom half. Tear the mozzarella slices into pieces on top of the mortadella. Season evenly with the salt and sprinkle with the capers. Close the sandwich with the top half of the baguette and cut into 4 or 6 pieces.
To fry the capers, warm 1/2 inch of grapeseed oil in a small saucepan over medium heat. When hot, add the capers and fry until opened, golden brown and crispy, about 2 minutes. Remove to a paper towel-lined plate to drain using a slotted spoon.