Use the pastry to line the base and sides of 2.5cm-deep, 26cm (base) fluted quiche pan. Trim excess (see tip). Place pastry case in the freezer for 15 minutes or until firm.
Meanwhile, heat the oil in a large non-stick frying pan over medium-high heat. Add the mushrooms and cook for 4 minutes or until light golden. Add the silverbeet and toss to combine. Remove from the heat. Stir in the onions. Set aside for 15 minutes to cool.
Place a baking tray in the oven. Preheat oven and tray to 200°C fan forced. Place the quiche pan on the hot tray and bake pastry case for 10–12 minutes or until light golden. Remove from oven. Reduce oven to 180°C fan forced.
Spread the mushroom mixture over the warm pastry base. Sprinkle with tasty cheese and feta. Using a fork, beat the eggs, cream and milk together in a bowl. Season with salt and pepper. Pour egg mixture over the cheese. Bake quiche for 25–30 minutes or until set in the centre. Serve warm or at room temperature.