Heat oil in a large saucepan over medium heat. Add onions and cook until lightly golden. Reduce heat; cook for 30 to 40 minutes until deep golden, stirring frequently.
Add broth and salt. Reduce heat to low; simmer for 15 to 20 minutes. Ladle soup into 4 oven-proof bowls; top each bowl with a toasted bread slice and sprinkle with cheese.
Broil 4 inches from heat for 2 to 3 minutes, until cheese is bubbly and lightly golden.