Don't limit this quick dish to weeknights; it makes a fabulous weekend brunch offering as well.
In a large cast-iron or other heavy skillet, bring potatoes to a boil in salted water over high. Cook until potatoes are tender when pierced with a knife, about 6 minutes. Drain.
In skillet, melt 2 tablespoons butter over medium. Add onion and bell pepper and cook until softened, about 10 minutes. Add potatoes and red-pepper flakes; season with salt. Cook until potatoes are golden, 3 minutes. Add spinach and 1 tablespoon butter and cook, stirring, until butter is melted. Make four wells in potato mixture. Crack 2 eggs into each well and reduce heat to medium-low. Cover and cook until whites are set but yolks are still runny, about 6 minutes. Sprinkle with Parmesan and parsley and serve immediately.
Use the back of a large spoon to hollow out wells for cooking the eggs. This helps them cook evenly and keeps portions separate for easy serving.