Combine the ground beef and onion in a cold skillet, place the skillet onto a stove burner, and turn the heat to medium; cook and stir until the beef is crumbly and browned. Stir the garlic and bell pepper into the beef mixture; continue cooking and stirring until the vegetables are tender, 2 to 3 more minutes. Add 1 cup of water. Mix in ketchup, brown sugar, Dijon mustard, Worcestershire, salt, and pepper. Pour in 1 more cup of water. Bring to a simmer. Reduce heat to low and cook until the mixture becomes very thick, 30 to 45 minutes. Adjust salt and pepper as needed. Season with cayenne.