Melt the butter in a Dutch oven over medium heat. Add the onion, peppers, carrots and celery and saute until the vegetables are soft.
Add the coriander, cumin, thyme, salt and pepper and flour, and stir to combine. Saute for a couple minutes to cook out the flour taste. Stir in the tomato paste, and then add the broth, milk, potatoes, chicken and corn to the pot.
Bring the mixture to a low boil, and then turn the heat down to low and simmer, with a lid on the pot, for about 15 minutes, stirring occasionally, until the potatoes are fork tender.
Stir in the lemon juice and cayenne pepper and then serve immediately.