Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with a bit of olive oil.
In a large skillet, heat a tablespoon of olive oil over medium heat. Add the chopped onion and garlic, sautéing until fragrant and the onions are translucent (about 3-4 minutes).
Add the chopped spinach to the skillet and cook until wilted (about 2-3 minutes). Remove from heat and let cool slightly.
In a large mixing bowl, whisk together the eggs, milk, oregano, salt, and pepper until well combined.
Stir in the wilted spinach mixture, crumbled feta, diced tomatoes, and diced bread into the egg mixture until fully incorporated.
Pour the mixture into the prepared baking dish, spreading it evenly.
Bake in the preheated oven for 30-35 minutes or until the casserole is set in the center and the top is golden brown.
Allow to cool for a few minutes before slicing into squares for serving.
Notes
Serve warm, garnished with fresh herbs and a side of mixed greens.