Step 1

Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Lightly grease a 9 x 12-inch (23 x 30 cm) baking dish.

Step 2

In a large frying pan, heat the olive oil over medium-high heat. Saute the diced red onions for about 5 minutes, or until they become lightly golden.

Step 3

Add the fresh spinach to the pan and sauté for an additional 2-3 minutes until the spinach wilts.

Step 4

Stir in the sliced kalamata olives and diced tomatoes into the vegetable mixture.

Step 5

Place the sliced pita bread into the prepared baking dish. Add the sautéed vegetable mixture over the pita bread, distributing it evenly. Top with crumbled feta cheese and shredded mozzarella cheese.

Step 6

In a separate bowl, whisk together the eggs, milk, dried oregano, salt, and pepper.

Step 7

Pour the egg mixture evenly over the ingredients in the baking pan.

Step 8

Allow the mixture to sit for 30 minutes, giving the liquid time to soak into the bread. Alternatively, you can cover and refrigerate it overnight for convenience.

Step 9

Bake the strata in the preheated oven for 40-50 minutes, or until it is puffed up and starting to brown on top.

Step 10

Let the strata sit for about 15 minutes before serving. This allows it to set and cool slightly.