Heat oil in a pan over medium heat. Add the minced beef, onions, and bell pepper, stirring frequently for 5-7 minutes until the beef is browned and crumbly. Pour off any excess liquid.
Add the garlic, mustard, ketchup, and brown sugar to the pan, stirring well to combine. Reduce heat to low and simmer for 30 minutes, stirring occasionally. Taste and season with salt and pepper to your preference.