Melt the margarine in a large soup pot over medium high heat, then add the onions (or leeks), carrots, and celery and sauté for about 6 minutes, until they are just starting to brown.
Add the mushrooms and sauté for about 4 minutes, then add the garlic (if using) and cook for 1 minute longer.
Add the broth, salt, and cassava flour and stir together.
Put the potatoes in the pot, bring it to a boil, then lower the heat to medium low and simmer, covered, until the potatoes are tender (about 12-14 minutes).
Add the chicken, bell pepper, and coconut milk and simmer for 5 more minutes.
Stir in the parsley and serve immediately.