Preheat your oven to 450 F.
Heat an ovenproof skillet over a medium heat and cook the bacon until crispy, about 7 minutes on each side.
Pat the cooked bacon dry and cut it into ¼-inch pieces.
Fry the onion in the rendered bacon fat you used to cook the bacon until translucent, about 4 minutes.
Turn off the heat, add the spinach and stir to slightly cook the spinach.
Whisk the eggs together with the water in a bowl until frothy and season to taste with salt and pepper.
Pour the egg mixture in the skillet with the onion and spinach and sprinkle the bacon pieces.
Stir once, turn the heat up to medium and cook until the frittata starts to set, about 5 minutes.
Place the skillet in the preheated oven and cook for 10 to 15 minutes, until the eggs are well cooked.