Heat the oil in a large skillet over medium-high heat. When hot, add the ground beef, breaking it up into large chunks, but leaving it alone otherwise for a minute or two to sear on the bottom. Once the ground beef has started to develop a crust on the bottom, continue to break it up and cook until browned through. Drain off excess grease, if necessary.
Add the onion, chopped bell pepper, and garlic. Season with salt and pepper, then cook for 5-6 minutes, stirring frequently, until softened.
Add tomato sauce, beef broth or water, ketchup, tomato paste, mustard, cider vinegar, Worcestershire sauce, brown sugar, chili powder, smoked paprika, ground cumin, and red pepper flakes. Mix until combined.
Reduce the heat to medium-low and simmer for 5-10 minutes, stirring occasionally, until thickened but saucy.
Serve on your favorite buns. A slice of cheese and pickles on top are optional, but delicious!