Preheat oven to 350° F. Coat a 12 count muffin tin with non-stick spray or line with silicone muffin cups.
Combine the eggs, heavy cream, diced onion and salt and pepper in a large bowl and whisk to combine, until the mixture is a pale yellow.
Divide the egg mixture between the 12 cups and add in your favorite toppings as desired, making sure to not fill the cups more than 3/4 of the way full.
Bake for 15 to 17 minutes, until the center of the egg cups are set and no longer shiny on top. Let them cool for a few minutes before removing from the cups. Enjoy right away or let cool completely, then wrap and store in the fridge.
Notes
Nutrition facts are based on the base egg mixture - add-ins will change the values.