Set oven rack about 15cm from the heat source and preheat the oven's grill on high. Line a baking tray with aluminum foil. Place chillies, cut sides down, onto the prepared baking tray.
Cook chillies under the preheated grill until the skin has blackened and blistered, 5 to 6 minutes. Place blackened chillies into a bowl and tightly seal with plastic wrap. Allow chillies to steam as they cool, about 15 minutes. Discard skins and chop chillies.
Coat a large frying pan with cooking spray over medium heat. Place prawns onto tortilla; cover with the chillies, garlic and mozzarella cheese. Season to taste with salt.
Fold tortilla in half and transfer to the pan. Cook until lightly toasted with spots of brown, 3 to 5 minutes per side.