Finely chop the onion. Heat oil in a large saucepan over medium-low or low heat. Add onion. Cook and stir occasionally while you continue.
While onion cooks, Chop garlic and bell peppers. Add to onions then cook and stir another 5 minutes.
Add broth, salt, pepper and red pepper flakes. Simmer uncovered until reduced by half, about 30 minutes. If needed, turn heat down to keep at a simmer.