Preheat oven to 325 ºF.
Heat a skillet to medium high and add chopped bacon. Cook for a few minutes, stirring.
Add sweet potatoes and onions. Add oil if the pan needs it.
Saute for about five minutes until potatoes are almost cooked through. Don't overcook at this point because don't want soft potatoes.
Remove from heat.
In a large bowl whisk eggs, milk, salt, and pepper.
Stir in spinach and potato mixture.
Spray a muffin tin with cooking spray and divide egg mixture evenly among cups (about ½ cup).
Cook for about 25 minutes or until they are firm.
These can be stored in the fridge in an airtight container for up to four days.