Preheat the oven to 400°F and grease a 9-inch pie plate.
Cook the ground beef and onion in a skillet over medium heat, stirring occasionally, until the beef has browned. Drain the beef, return it to the pan, and add the taco seasoning. Mix well. Spoon the beef evenly into the pie plate and top with green chilies.
In a mixing bowl, whisk the milk, eggs, and Bisquick mix until blended. Pour the mixture over the ground beef in the pie plate.
Bake for about 25 minutes or until a knife inserted into the center comes out clean. Sprinkle with cheddar cheese and bake for an additional 8 to 10 minutes. Remove from the oven and let cool for 5 minutes.
Top with shredded lettuce, chopped tomato, and sour cream.