Step 1: Preheat oven to 350°F (175°C).
Step 2: In a bowl, whisk together eggs, milk, salt, pepper, and oregano until well combined.
Step 3: Heat olive oil in a 10-inch oven-safe skillet over medium heat.
Step 4: Add onion and garlic to the skillet, sauté for 3-4 minutes until softened.
Step 5: Stir in chopped spinach and cook for 2 minutes until wilted.
Step 6: Pour the egg mixture over the vegetables in the skillet.
Step 7: Sprinkle shredded cheddar cheese evenly over the top.
Step 8: Cook on the stovetop for 5 minutes without stirring.
Step 9: Transfer the skillet to the preheated oven and bake for 20-25 minutes until set and golden.
Step 10: Let cool for 5 minutes before slicing and serving.