Preheat oven to 350 degrees F.
Brown ground beef in a large skillet over medium heat, then drain grease.
Add onion, pepper, and garlic and heat for an additional 5 minutes or until onions are translucent.
Add ketchup, brown sugar, Worcestershire sauce, mustard powder, salt, and pepper, and mix until combined and bring to a simmer over medium heat.
Reduce heat and simmer for 8-9 minutes.
Mix half of shredded cheese into sloppy joe mixture.
Spread into 9-inch cast iron skillet or and 8x8-inch baking dish.
Top with remaining shredded cheese.
Bake in preheated oven for 18-19 minutes or until bubbly and cheese is beginning to brown.
Allow to cool slightly, top with optional green onion slices if desired, and serve.