In a skillet, cook the ground beef and onion over medium heat until the beef is browned and the onion is softened. Drain any excess grease.
Stir in the taco seasoning mix and cook for an additional 2-3 minutes, until well combined. Remove from heat and spread the beef mixture evenly in the bottom of the prepared pie dish.
Sprinkle the shredded cheddar cheese over the beef mixture.
In a mixing bowl, whisk together the Bisquick mix, milk, and eggs until smooth. Pour the mixture over the beef and cheese in the pie dish.
Bake in the preheated oven for 25-30 minutes, or until the pie is set and the top is golden brown.
Remove from the oven and let it cool for a few minutes before slicing.
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