Preheat oven to 200 C / Gas 6. Bake pastry case in preheated oven for 8 minutes.
Meanwhile, heat olive oil in a large frying pan over medium heat. Saute onion until soft; remove from pan. Sprinkle tomato slices with flour and basil, then saute 1 minute on each side. In a small bowl, whisk together eggs, Parmesan and milk. season with salt and pepper.
Spread 2/3 of the grated cheese in the bottom of pastry case. Layer onions over cheese, and top with tomatoes. Cover with egg mixture. sprinkle top with remaining cheese.
Bake in preheated oven for 10 minutes. Reduce heat to 180 C / Gas 4, and bake for 15 to 20 minutes, or until filling is puffed and golden brown. Serve warm.