Pre-heat the oven to 160ºC.
Spray a frying pan with low calorie cooking spray and place over a low heat. Add the onion and fry gently for 10 minutes, stirring occasionally.
Add the garlic to the onions and fry for a further 5 minutes, until the onions are softened and golden. Set aside.
Spray the holes of a muffin tray with a little low calorie cooking spray and use to grease well.
Take a square of filo pastry and brush with egg on one side. Place the pastry square, glazed side up, in a muffin hole. Press gently into the muffin tray to form a pastry case.
Repeat this with a second pastry square, placing at an angle to the first.
Continue to repeat this process until all 12 pastry cases are complete. Keep the leftover egg as this will be added to the cheese mixture later.
Place the tray into the preheated oven for 3 - 4 minutes, to partly bake. The egg glaze will have just dried out and the pastry will still be mostly white, with it turning a light golden in places.
Remove the pastry cases from the oven and set aside.
Place the cooked onion and garlic in a small mixing bowl. Stir in the quark, Parmesan cheese, and basil, and then add the remaining egg and season well with salt and pepper.
Divide the cheese mixture between the 12 partly cooked pastry cases and gently spread it out.
Place a halved cherry tomato on top of each tart and return to the oven for 8 - 10 minutes, until the pastry is golden and the filling just set. Take care to watch the tarts carefully as they will cook very quickly and can burn easily.
Remove from the oven and leave in the muffin tray until they are cool enough to handle. Garnish each with a small basil leaf and serve warm or cold.