Melt 30 g butter with the olive oil in a 5-quart / 4.7 L heavy pot (24 cm / 9.5-inch) over medium heat. Add the onion, carrot, celery, 2 sage leaves and 1/2 tsp salt. Cook for 8 minutes, stirring, until soft and pale gold but not browned.

Add the potatoes and cabbage and stir for 5 minutes until the cabbage has wilted by about half. Add the beans and 1.6 L water, bring to a boil, then lower to a gentle simmer. Partly cover and cook for 20 minutes until the potatoes are tender when pierced.

Turn the heat to low so the surface barely bubbles. Whisk in the cornmeal in a thin stream, a handful at a time. Dumping it in at once makes lumps that won't break up later.

Switch to a flat wooden spoon and cook for 25 minutes, stirring and scraping the bottom every 3 to 4 minutes, until the soup is thick and the cornmeal tastes sweet with no grit. Loosen with 100 ml hot water at a time if it gets stiff. Stir in 1 tsp salt and the pepper, then taste.

Melt 20 g butter in a 20 cm / 8-inch skillet over medium heat. Add 6 sage leaves and cook for about 2 minutes until the leaves crisp and the butter smells nutty.

Ladle the soup into wide bowls, scatter the Casera over the top and spoon on the sage butter. Serve hot.