Pour the chicken, beef, or vegetable broth into a 2-quart / 1.9 L saucepan and bring it to a bare simmer, then keep it hot over low heat. Finely chop the onion and cut the Casera into 1 cm cubes.
Melt 20 g butter in a 28 cm / 11-inch heavy-bottomed sauté pan over medium-low heat. Add the onion and the salt, then cook for 5 minutes, stirring, until soft and translucent but not browned.
Add the rice and stir for 2 minutes until the grains turn translucent at the edges and smell nutty. Pour in the wine and stir for about 1 minute until it's absorbed.
Add the hot broth about 150 ml at a time, stirring often over medium heat so the mixture stays at a gentle bubble. Add the next ladle when the pan is nearly dry. Keep going for 16 to 18 minutes, until the grains are tender with a slight bite at the center and the rice flows slowly when you tilt the pan. Keep the last ladle of broth in reserve.
During the last 5 minutes of cooking, melt 30 g butter in an 18 cm / 7-inch skillet over medium heat. Add the sage leaves and cook for 2 to 3 minutes until the butter foams and smells nutty and the leaves turn crisp. Remove from the heat.
Take the rice off the heat and wait 30 seconds. Stir in the Casera, the 10 g cold butter, and the reserved broth until glossy, then cover and rest for 2 minutes. Season with pepper and adjust the salt, then spoon into warm bowls and top with the sage butter. Serve right away.