Preheat the oven to 400°F (200°C). Grease a 9-inch pie dish.

Heat a skillet over medium. Add the ground beef and diced onion. Cook, breaking the meat up, until browned and the onion is tender, about 6–8 minutes. Drain excess fat.

Return the meat to the pan, season with salt and pepper, and stir to combine.

Spread the beef and onion mixture evenly in the prepared pie dish. Sprinkle the shredded cheddar over the meat.

In a medium bowl, whisk together the Bisquick (or flour mix), milk, and eggs until smooth.

Pour the batter gently over the meat and cheese, tilting the dish as needed to cover evenly.

Bake for 25 to 30 minutes, until the top is golden and a knife inserted comes out clean.

Remove from the oven and let the pie rest for 5 minutes before slicing.