Begin by preparing the tartlet dough, which requires just two simple ingredients: 1⅓ cups all-purpose flour and 6 tablespoons butter.
In a bowl, rub the flour and butter together using your fingertips until the mixture resembles breadcrumbs.
Add approximately 2 tablespoons of cold water gradually, mixing until the dough comes together in a soft, cohesive ball.
Wrap the dough in plastic wrap and refrigerate for 30 minutes to allow the gluten to relax and the butter to firm up, which will result in a crispier final product.
Once chilled, preheat your oven to 400°F (200°C). On a lightly floured work surface, roll out the tartlet dough to a thickness of ⅛–¼ inch (3–6mm).
Using a 4-inch pastry cutter or the rim of a glass or saucer, cut out 12 rounds from the dough. Carefully fold up the edges of each round to create small rims, then arrange them on a baking sheet.
Spread ⅓ cup of store-bought red or green pesto evenly into the bottom of each tartlet shell, then fill with the remaining toppings: 1¼ cups of finely chopped ripe tomatoes, 2–3 crushed garlic cloves, 9 black olives (pitted and quartered), 4 ounces of grated Fontina or mozzarella cheese, 2–3 tablespoons of grated Parmesan cheese, and 2 teaspoons of chopped fresh marjoram distributed across all the tartlets.
Bake the tartlets in the preheated oven until the pastry is golden brown and crispy, and the cheese is melted and bubbling. The exact baking time will depend on your oven, but typically tartlets of this size require 12–15 minutes.
Once baked, remove from the oven and allow to cool slightly before serving warm as an appetizer or light meal.
