Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius) and line a baking tray with parchment paper.
In a food processor or with a hand blender, blitz the olives, garlic, thyme, olive oil, and vinegar together to form the tapenade.
Roll out the puff pastry onto the parchment paper and, with a knife, score a two-centimeter border around the pastry so it looks somewhat like a picture frame.
Spread the tapenade inside the border of the pastry and layer the tomatoes, mozzarella, and onion over.
Bake for 25 to 30 minutes until the sides of the pastry turn golden. Remove from oven and serve warm.