Preheat the oven to 350˚F (180˚C). Grease a 6-cup muffin tin with nonstick spray.
Heat the oil in a large skillet over medium heat until shimmering. Add the onion and sauté for 4-5 minutes, until the onion is translucent. Add the garlic and cook for another 1-2 minutes until fragrant.
Add the ground beef and cook, breaking up with a wooden spoon, until browned, about 5 minutes. Drain any excess fat.
Add the mustard powder, pepper, and salt. Stir to combine, then add the tomato sauce, and cook for 5 minutes until liquid is cooked down.
Add the Worcestershire sauce and ketchup and cook for 10 minutes, until most of the liquid has evaporated. Remove the pan from the heat.
Slice the crusts off the bread. Quarter 2 of the slices to make small squares, leaving the remaining 6 slices whole.
Press the whole slices of bread into the cups of the greased muffin tin, and brush with melted butter. Add 1 tablespoon of shredded cheddar cheese to each cup, and top with a quartered piece of bread. Brush the bread with melted butter.
Bake for 10 minutes, until the cheese melted.
Scoop 2 tablespoons of sloppy joe mixture into each cup, and top with another tablespoon of cheddar.
Bake for another 5 minutes, until the cheese is melted and the cups are firm.