Melt 40 g butter in a 26 cm / 10-inch (4.7 L / 5-quart) Dutch oven over medium-low heat. Add the onion, 4 thinly sliced sage leaves and a pinch of salt, and cook for 8 minutes, stirring, until soft and pale gold with no browning. Add the garlic for the last minute.
Stir in the potatoes and cabbage and cook for 5 minutes, stirring often, until the cabbage has wilted by about a third and everything is coated in butter.
Pour in the hot broth, add the pepper and salt, and bring to a boil. Reduce to a gentle simmer, partly cover, and cook for 15 minutes until the potatoes are tender when pierced with a knife but still hold their edges.
Stir in the beans and ditalini. Simmer uncovered for 9 to 10 minutes, stirring often so the pasta doesn't stick, until the ditalini is al dente and a spoon leaves a trail. If the pot looks tight, add up to 100 ml hot water.
Take the pot off the heat, stir in 70 g of the Casera, cover, and rest for 5 minutes. Taste and adjust the salt.
During the rest, melt 20 g butter in a 20 cm / 8-inch skillet over medium heat. Add the remaining 4 sage leaves and cook for 2 to 3 minutes until the butter foams, smells nutty, and the leaves crisp. Ladle the soup into bowls, spoon over the sage butter, and top with the remaining 30 g Casera.