In a large skillet over medium-high heat oil. Sauté the onion and red bell pepper until onions turns translucent. Add garlic and cook for 1 to 2 minutes. Add ground beef and sauté until browned, about 5 to 7 minutes. Drain any excess fat.
Add tomato paste, stirring to combine. Cook for 2-3 minutes to remove raw tomato flavor. Add in stock, chili powder, paprika, cumin, vinegar, brown sugar, oregano, cayenne, salt and pepper, and stir to combine. Simmer uncovered 5 to 10 minutes, or until mixture reaches desired thickness. Serve on toasted hamburger buns.