Warm the pot: Heat olive oil in a large Dutch oven or heavy pot over medium heat. Add the diced onion and red bell pepper.
Cook 4–5 minutes, stirring occasionally, until softened.
Add garlic and tomato paste: Stir in the garlic and cook 30 seconds until fragrant. Add tomato paste and cook 1 minute, stirring, to lightly caramelize it.
Brown the beef: Add the ground beef. Cook 5–7 minutes, breaking it up with a spoon, until no longer pink.
Spoon off excess fat if needed for a lighter chili.
Bloom the spices: Sprinkle in chili powder, cumin, smoked paprika, oregano, red pepper flakes (if using), salt, and black pepper. Stir for 30–60 seconds so the spices toast and become aromatic.
Build the base: Stir in crushed tomatoes, fire-roasted tomatoes, lentils, and broth. Bring to a gentle boil over medium-high heat.
Simmer patiently: Reduce heat to low, cover partially, and simmer 30–40 minutes, stirring occasionally, until lentils are tender but not mushy.
Add more broth if you want a looser texture.
Brighten and adjust: Stir in apple cider vinegar or lime juice. Taste and adjust salt, pepper, and chili powder. If you like more heat, add a pinch of cayenne or extra red pepper flakes.
Serve and top: Ladle into bowls and finish with your favorite toppings.
Greek yogurt or avocado balances the spice; cilantro and lime add freshness.