Preheat and prep the pan. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy lifting. Lightly oil the parchment and sides.

Sweat the aromatics. Warm 2 tablespoons olive oil in a skillet over medium heat. Add the onion and cook 3–4 minutes until soft. Stir in garlic and cook 30 seconds until fragrant.

Cook the veggies lightly. Add bell pepper, zucchini, and broccoli. Cook 3–5 minutes until just tender and moisture has mostly evaporated. Stir in chopped spinach and cook 1 minute to wilt. Season with a pinch of salt and pepper. Remove from heat and let cool slightly.

Whisk the eggs. In a large bowl, whisk eggs with milk, oregano, pepper, and a small pinch of salt. For a fluffier texture, whisk until slightly frothy.

Combine. Fold the cooled vegetables and cheese (if using) into the egg mixture. Taste a small spoonful (raw eggs—don’t swallow) to gauge seasoning, then adjust salt and pepper.

Pour and smooth. Pour into the prepared pan and spread evenly. Tap the pan gently on the counter to release air bubbles.

Bake. Bake 18–24 minutes, until the center is set and the top is lightly golden. A knife inserted near the center should come out clean.

Cool and slice. Let the frittata cool in the pan for 10–15 minutes. Use the parchment overhang to lift it out. Slice into finger-length strips or small rectangles.

Serve. Enjoy warm or at room temperature. Pair with a simple dip like yogurt-herb sauce, salsa, or a swipe of hummus.