Heat your olive oil in a large frying pan over medium heat. Add the ground beef and leeks and cook until browned. Drain any liquids and set aside.
Place green peppers in the same pan and cook until tender (about 3-4 minutes). Return the beef to the pan and add the ketchup, tomato paste, water, brown sugar, yellow mustard, chili powder, Worchestershire sauce, and red pepper flakes.
Reduce the heat to medium-low and simmer for 30 minutes. Once the sauce has thickened, season it with salt and pepper to taste, then divide the mixture between 6 hamburger buns and serve immediately.
Notes
Recommended low FODMAP serving – 1/2 a cup of sauce per sitting