Add ½ cup chopped onion, 3 minced garlic cloves, and ½ lb ground beef to a large pot over medium-high heat. Break apart the ground beef with a flat wood spoon. Stir and cook until the beef has browned, about 2-3 minutes.
Add in the 14 oz canned diced tomatoes (with the liquid from can), 6 oz tomato paste, 4 cups water, 15 oz canned kidney beans (drained), 1 tablespoon chili powder, ½ teaspoon salt and ¼ ground black pepper. Stir together. Bring to a boil, then reduce heat to let simmer for about 25-30 minutes. Stir frequently.
While soup is simmering, mix together the 1 tablespoon cornstarch and ¼ cup of water in a small bowl.
Pour the cornstarch mixture into the soup in the last 10 minutes. Stir together and let the soup simmer until thickened. Remove from heat and serve.