Cook the beef and onion in a Dutch oven over medium-high heat for 8 to 10 minutes until the beef reaches 160°F.
Drain excess fat, stir in the chili powder and cumin for 30 seconds, then add the tomatoes, kidney beans, broth and salt.
Simmer uncovered for 30 minutes until the chili is thick enough to mound in a spoon.
Divide the hot chili among 6 broiler-safe bowls and scatter 1/4 cup cheddar evenly over each bowl.
Broil the bowls 5 inches from the heat for 2 to 3 minutes until the cheese bubbles and develops brown patches, then add the measured scallions and rest for 3 minutes.