Cook the Bacon: Preheat your oven to 375°F (190°C). In a skillet over medium heat, cook the bacon pieces until crispy. Remove the bacon and set it aside, leaving a small amount of grease in the skillet.

Sauté the Vegetables: Add the diced onion to the skillet and sauté until softened, about 3-4 minutes. Add the fresh spinach and cook until wilted, stirring occasionally. Remove from heat and let it cool slightly.

Prepare the Egg Mixture: In a mixing bowl, whisk the eggs until fully beaten. Stir in the feta cheese crumbles, cooked bacon, and the sautéed spinach and onion. Season with salt and pepper to taste.

Bake the Egg Dish: Pour the mixture into a greased baking dish (approximately 8x8 inches). Spread it evenly and bake in the preheated oven for 20-25 minutes, or until the eggs are set and the top is slightly golden.

Serve and Enjoy: Let the egg bake cool slightly before slicing. Serve warm, and enjoy this protein-packed breakfast on its own or with toast or a fresh salad.