Preheat oven to 400°F (200°C). Lightly grease a 9-inch pie dish.
In a skillet over medium heat, cook the ground beef and chopped onion until beef is browned and onion is tender. Drain excess grease.
Stir in taco seasoning, green chilies, salt, and pepper. Mix well and spread evenly in the prepared pie dish.
Sprinkle shredded cheddar cheese evenly over the beef mixture.
In a mixing bowl, whisk together Bisquick, milk, and eggs until smooth. Pour over the beef and cheese.
Bake for 25–30 minutes, or until a knife inserted in the center comes out clean.
Let stand 5 minutes before slicing. Top with sour cream and diced tomatoes if desired.