Preheat oven to 350°F (175°C).
Lightly grease a 9x13-inch baking dish.
Heat olive oil in a skillet over medium heat.
Cook ground beef and chopped onion until browned.
Stir in taco seasoning, salt, and black pepper.
Spread cooked beef mixture evenly in baking dish.
Sprinkle green chilies and diced tomatoes over beef.
In a bowl, whisk eggs and milk until combined.
Pour egg mixture evenly over beef layer.
Top with cheddar and Monterey Jack cheese.
Bake for 35–40 minutes until eggs are set and top is golden.
Garnish with fresh cilantro before serving.