In an oven-safe skillet or baking dish, heat 1 tbsp olive oil over medium heat. Add 1/4 cup finely diced red onion and 1/2 cup diced red bell pepper. Sauté for 3-4 minutes until softened.
Stir in 1 cup halved cherry tomatoes and cook for another 2 minutes.
Add 2 cups of fresh spinach to the skillet, stirring until just wilted. Remove from heat.
Sprinkle 4 oz of crumbled feta cheese evenly over the vegetable mixture. Season with 1/2 tsp salt and 1/4 tsp black pepper.
Use a spoon to create 4 small wells in the mixture. Carefully crack 1 large egg into each well.
Transfer the skillet to the preheated oven and bake for 10-12 minutes, or until the egg whites are set but the yolks are still runny.
Remove from the oven, garnish with 1 tbsp chopped fresh chives, and serve immediately with 4 slices of crusty bread.