Preheat the oven to 375° F. Grease pie plate, and set aside.
Cook the bacon in a frying pan until crisp, drain, then cool on a paper towel lined plate. Roughly chop once cool.
Melt butter over medium heat in a saute or sauce pan, then add onion. Season to taste with salt and pepper, then cook until onion has softened and is starting to brown. Remove heat, and set aside.
In a large bowl, whisk together cream, milk, and eggs. Add bacon, onions, cheese, salt, and pepper until well combined. Pour into prepared pie plate. (Make sure you do not fill to the rim - if you have too much filling, pour into a custard cup and bake beside the frittata, or save for later.)
Bake for 45-60 minutes. The edges should be golden brown, and the center just set.
Remove frittata from the oven, and let cool on a rack. Serve warm or at room temperature, and refrigerate the leftovers.
Notes
The finished egg mixture can be placed in an airtight container (NOT the pie plate) and saved for up to 3 days. Pour into prepared pan, and bake in preheated oven - baking time will of course be extended.