Put the flour, salt and sugar in a food processor. Add cubes of cold butter and pulse 7-8 times to form small beads in the flour. Check the uniformity of the flour mixture and cut any large pieces of butter with a knife. Pour water and pulse 7-8 times to just combine the ingredients. Dump the mixture onto a cold work surface and lightly knead with heal of your palm for 2 minutes. Be careful to not overwork the dough (good dough is crumbly and cold). Collect the dough and tightly wrap in plastic. Refrigerate the dough for at least 30 minutes.

Meanwhile, boil a pot of water. Dip the tomato half way in boiling water, for less than a minute. Remove the tomato and cut a shallow cross on the bottom. When the tomato cools slightly, peal off the skin and slice the tomato cross wise.

Heat oil in a shallow saucepan, add garlic and cook until the garlic turns light brown. Sauté the onions and capsicum and add the parsley and basil. Turn off the heat and let the veggies cool.

Preheat oven to 175 °C/350°F and line a baking tray with parchment (baking) paper. Remove the dough from the refrigerator and leave at room temperature for 5 minutes. Place the chilled pastry dough on to lightly floured work surface and fold it 2-3 times to make it pliable. Dust some flour on a rolling pin and begin rolling the dough from the center to the edge. Turn the dough 90 degrees after each rolling action. Continue to roll and turn, till the dough is an even disc with 10-inch diameter. Brush off any excess flour from the dough and carefully transfer the base on to the prepared baking tray. Trim off the uneven edges and fold 1/2-inch of the edge inward. Prick the center with a fork a few times and bake until firm, about 10-12 minutes. Remove the base from the oven and let it cool on a wire rack for a few minutes.

Place the base on the baking tray. Spread the onion mixture on to the base and arrange the tomato slices and black olives. Sprinkle the cheese, salt and pepper. Bake until the crust is golden brown, about 13-15 minutes.

Remove from the oven and transfer to a cutting board. Slice the galette using a serrated knife and serve immediately.