Using a colander held under a cool tap, rinse the grated sweet potatoes until the water runs mostly clear. Squeeze the rinsed gratings in a clean tea towel to remove excess water, then tip them into a large mixing bowl.
Add the onion, garlic, ginger, both flours, the chopped chilli, 1 tbsp of the coriander, 2 tbsp of the oil or melted coconut butter and seasoning and mix everything well.
Divide the remaining oil or coconut butter between 4 x 15cm diameter cast-iron or heavy-based frying pans – or a 23cm diameter pan to make a large cake – and place over a high heat. Add one-quarter of the potato mixture to each pan, or all of it to the large pan (it should sizzle loudly) and flatten firmly with a spatula. Reduce the heat to medium.
Cook each cake for 3-5 minutes (slightly longer for the larger version), then run a palette knife or spatula around the edge, tip each cake out on to a flat plate and slide the cake back into its pan the other way up. (Don’t attempt to flip these unless you’re a black belt.) Return to the heat for about 3 minutes more until the underside is nicely browned.
Scoop the avocados out of their skins with a spoon and smash roughly with a little lime juice, the remaining coriander, sliced chilli and seasoning to taste. Serve each hot sweet potato cake with the avocado mixture spooned on top and lime quarters to squeeze over.