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.
Nutrition facts are based on the base egg mixture - add-ins will change the values.