Pour the marinara sauce into a medium skillet and bring it to a gentle simmer over medium heat.
Stir in the chopped spinach and cook for 1–2 minutes, just until wilted.
Use a spoon to make four small wells in the sauce. Crack one egg into each well.
Cover the skillet and reduce the heat to medium-low. Cook for 5–7 minutes, or until the egg whites are set and the yolks reach your preferred doneness.
Scatter the sliced mozzarella over the top, cover again, and cook for 1–2 minutes, until the cheese is melted.