Heat up a panini press or a skillet with enough olive oil to lightly coat the skillet. Take the bun and cut it in half.
Spread the mayo on both pieces.
Take a few spinach leaves and press/ lay flat on each piece. (Make sure they stick!!)
Take your fish and either: Cut into pieces and lay it on one bun piece
OR break up the fillet with a fork and pile and flatten it onto the bun piece.
Lay the mozzarella slices on top of the fish.
Complete with the other bun piece- be careful that the spinach doesn't fall off!
Panini or grill until the mozzarella is melted and the bun is golden brown