Heat a large saucepan or Dutch oven over medium-high heat. Add the lean ground beef or turkey and chopped onion, then cook for about 5 to 7 minutes while breaking the meat into small pieces.

Continue cooking until the meat is browned throughout and the onion is softened. Drain excess fat if necessary, especially when using ground beef, so the finished chili stays flavorful rather than greasy.

Stir in the drained kidney beans, undrained diced tomatoes, tomato sauce, chili powder, cumin, salt, and black pepper. Add the optional chopped green pepper or cayenne at this stage if desired.

Bring the chili to a gentle boil over medium-high heat. Once bubbling, reduce the heat and simmer uncovered for 8 to 10 minutes, stirring occasionally so the ingredients combine evenly.

Taste the chili and adjust the chili powder, salt, and black pepper to suit your preference. For fuller flavor, continue simmering for 15 to 20 minutes when time allows, then serve hot with optional shredded cheese or sour cream.