Preheat oven to 200 degrees C. Prepare pastry base with ready made shortcrust pastry.
Melt butter or oil in a large frying pan over medium-low heat. Stir in sliced red onion; cook, stirring occasionally, for about 5 -8 minutes or until slightly soft. Season with salt and pepper. Reduce heat to low. Stir in cream and Stilton and warm through. Pour mixture into pastry base.
Bake in preheated oven for 30 minutes, or until quiche has set and is golden brown on top. Allow to sit 10 minutes before slicing.