01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper or grease it thoroughly.
02 - If using fresh spinach, heat olive oil in a skillet over medium heat. Sauté onion and bell pepper for 3 to 4 minutes until softened. Add spinach and cook until wilted. Remove from heat and allow to cool. If using frozen spinach, drain excess moisture and combine with sautéed onion and bell pepper.
03 - In a large bowl, whisk together eggs, milk, salt, black pepper, and dried oregano until fully blended.
04 - Add shredded cheddar, grated Parmesan, cooled vegetables, flour, and baking powder to the wet mixture. Stir gently until just combined.
05 - Pour the batter evenly into the prepared pan and smooth the surface. Bake for 25 to 30 minutes until set and golden on top.
06 - Allow bars to cool slightly before cutting into 12 portions. Serve warm or at room temperature.