For the salsa verde, pick the leaves from the basil and parsley bushes. Puree the herb leaves with 200 ml of olive oil and the vinegar, mustard, gherkins and capers in a blender or with an immersion blender until you have a smooth groos-green sauce. Store the salsa in a cool place.
Preheat the oven to 220 °C. Place the gratins on the baking tray and divide the courgette and cheese on top. Bake the gratins in the preheated oven for 25 minutes until golden brown and cooked.
Divide the gratins among 4 plates. Spoon some salsa verde on the side.