Heat the oven to 190°C/Fan 170°C/Gas 5. Pour 200ml boiling water into a large pan. Bring to a bubble then trickle in the polenta, stirring. Add 1 tsp ground cumin and plenty of salt and pepper. Cook for 5 mins till thick. Using a couple of teaspoons, dollop 10-12 little mounds onto a lined baking tray. Bake for 15 mins till golden. Turn the oven off and keep them warm inside.
While the polenta bites are baking, peel and finely chop the onions. Peel and grate or crush the garlic. Halve the chilli, flicking out the seeds for less heat. Keep the seeds and you can add them to taste later. Crush the coriander seeds in a pestle and mortar till you have a coarse powder. Finely chop the coriander stalks, setting the leaves aside for later. Halve the tomatoes.
Heat a large heavy based pan for a couple of mins. When warm, drizzle in 1 tbsp olive oil. Add the onions and fry for 5 mins till softened. Add in the garlic, chilli, coriander stalks and seeds and the tomatoes. Cook for 3-4 mins till the tomatoes have softened and the pan smells spicy.
Pour 400ml boiling water into the pan. Lower the heat and simmer for 5 mins.
While the tomatoes are simmering, drain and rinse the beans. Halve the avocado and scoop out the flesh. Pop it into a bowl and mash it with the back of a fork. Zest the lemon and set that aside. Squeeze a little of the juice into the avocado. Season generously. Taste and add more lemon or seasoning if it needs it.
After the tomatoes have simmered for 5 mins, add the drained beans to the pan. Turn the heat up and cook for 10 mins. The liquid should have reduced by half. Remove a spoonful of the beans and liquid into a bowl. Crush with the back of a fork. Tip back into the pan. This will thicken the chilli a little.
Add the lemon zest and juice from half a lemon to the pan. Taste the white bean chilli and adjust the seasoning if you think it needs it. Add a few chopped chilli seeds for more of a kick.
Ladle into warm bowls. Drop in a few polenta bites. Top with the smashed avocado and coriander leaves to serve.