Preheat oven to 350°F (180°C)
Line a muffin tin with 12 muffin liners or spray with oil.
Place a skillet over medium heat and add the butter and oil.
When hot, add the onions and sauté for about 3 minutes, stirring frequently, until onions are lightly browned.
Add the red peppers and sauté for 1 to 2 minutes or until soft.
Allow the mixture to cool slightly.
While the vegetables are cooking, crack the eggs into a medium mixing bowl and add the water.
Whisk until well combined.
Mix in the parmesan cheese, oregano, salt and spinach.
Set aside.
Add the cooked onions and peppers to the bowl.
Fold together until well blended.
Using a ladle, pour the egg mixture into the muffin tins to 3/4 of the way.
Sprinkle the top of egg mixture with feta cheese.
Place the muffin tins into the oven and cook for about 20 to 25 minutes, until egg is firm.
Allow frittata to rest for 5 minutes, and then scoop the individual muffins out onto a platter.
Serve warm or cold and enjoy!