Preheat your oven to 400 degrees F. Place a 10-inch oven-safe skillet, preferably cast iron, over medium-high heat on the stovetop.

Add the ground beef and diced onion to the hot skillet. Cook, breaking the meat apart with a wooden spoon, until the beef is nicely browned and the onion is soft and translucent. Drain off any excess grease.

Add the minced garlic, ketchup, mustard, Worcestershire sauce, and pickle relish to the skillet. Stir everything together and let it cook for about a minute until it is bubbling and fragrant.

Sprinkle the shredded cheddar cheese evenly over the hot beef mixture. The residual heat will begin to melt the cheese slightly, creating a glorious, gooey layer.

In a medium bowl, whisk together the milk, eggs, flour, baking powder, salt, and pepper until smooth. The batter should be pourable, similar to a thin pancake batter.

Pour the batter evenly over the cheese and beef mixture in the skillet. Do not stir it. Simply let the batter settle on top. Carefully transfer the skillet to the preheated oven. Bake for 25 to 30 minutes, or until the top is puffed, deeply golden brown, and a toothpick inserted into the center comes out clean.

Let the pie rest in the skillet for at least 5 to 10 minutes. This crucial step allows the layers to set, making it easier to slice and serve. Scoop out generous portions and serve.