1. Preheat your oven to 375°F (190°C). Lightly coat a 48-cavity mini muffin tin with cooking spray and set it aside.
2. In a large bowl, whisk together the flour, baking powder, and salt until well combined. Create a space in the center of the dry ingredients.
3. In a separate bowl, whisk the vegetable oil, milk, and eggs. Pour this wet mixture into the space you made in the large bowl. Stir everything together just until the dry ingredients are moistened. Do not overmix.
4. Gently fold in the ½ cup of ketchup, 2 tablespoons of mustard, and 1 cup of shredded cheddar cheese into the batter. Set this batter aside while you prepare the meat.
5. Heat a large skillet over medium heat. Add the ground beef and the diced onion. Cook this mixture until the ground beef is fully cooked through and the onion has softened.
6. Take 1 cup of the cooked ground beef and onion mixture and add it to your muffin batter. Stir it in gently to combine.
7. Spoon the batter into the prepared mini muffin tin cavities until they are almost completely full.
8. Bake for about 15 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Carefully transfer the baked muffins to a wire rack to cool slightly.

For the Filling:

1. Place the remaining ground beef mixture into a small bowl.
2. Stir in the 4 ½ tablespoons of ketchup, 1 tablespoon of mustard, and the diced dill pickles until everything is mixed together.
3. Once the muffins have cooled enough to handle, use an apple corer or a small paring knife to carefully remove the center of each muffin. Be sure not to cut all the way through the bottom.
4. Using a small spoon, fill each hollowed-out muffin with the ground beef filling mixture.
5. Place the filled muffins back into the mini muffin tin. Sprinkle the tops of the filled muffins with the remaining ¼ cup of shredded cheese.
6. Return the muffin tin to the preheated 375°F (190°C) oven for about 3 to 4 minutes. This brief bake time will heat up the filling and melt the cheese on top.
7. Serve the muffins hot. Offer extra ketchup and mustard alongside for dipping, if desired.