Heat the oil in a large soup pot over a medium heat. Add the onion and garlic, and saute for 5 minutes until translucent. Add the red pepper flakes and cook, stirring, for 1 minute more.
Turn the heat up to high. Add the plum tomatoes and the balsamic, and stir to combine with the onions.
Cook at a rapid simmer, stirring infrequently, until tomatoes are slightly caramelised at the bottom of the pan- about 5 minutes.
Add the stock and split peas. Bring to a simmer and cover. Simmer for 30 minutes, stirring occasionally to stop the peas from sticking to the bottom.
After half an hour, add the chopped bell peppers. Cover again and simmer for 10 minutes.
Add the courgette, aubergine and broad beans to the pan. Season and stir, then cover and simmer for a final 15 minutes.
Stir in the lemon juice and herbs, and adjust seasoning to taste.