Preheat oven to 375°F and grease a 10-inch ovenproof skillet with olive oil.

Heat olive oil over medium; add red onion, bell pepper, zucchini, and garlic. Cook 5 minutes until softened and lightly golden.

Stir in sun-dried tomatoes and baby spinach; cook 1–2 minutes until spinach wilts.

In a bowl, whisk large eggs with whole milk, salt, and pepper until frothy, about 1 minute.

Pour egg mixture over veggies in skillet; stir gently, then let sit 1 minute until edges start to firm.

Bake 20–25 minutes until puffed and golden. Remove, let rest 5 minutes, then slice and garnish with fresh oregano.