In a heavy based saucepan over a medium heat gently fry the onions and garlic in the olive oil for 2-3 mins until soft and just starting to caramelise. Add the chipotle paste to the pan and cook through for a few minutes before adding the beans and chopped tomatoes, simmer for 8-10 mins until thickened, season to taste and remove from the heat and finish with the
coriander.
Under a high grill toast each side of the sourdough before topping with the beans and cheese and returning to the grill under melted and bubbling. Serve with a dollop of sour cream and avocado slices