Trim the tough stems from the turnip greens, wash well, and roughly chop the leaves and tender stems.
Bring a pot of salted water to a boil and blanch the greens for 2 to 3 minutes until just wilted. Drain and set aside.
In a 26 cm / 10-inch heavy pot or Dutch oven, melt 40 g of the butter over medium heat. Add the onion and cook for 5 to 6 minutes until soft and translucent.
Add the garlic and cook for 30 seconds until fragrant.
Pour in the hot broth and bring to a gentle simmer.
Stir in the rice, partially cover, and simmer for 12 to 15 minutes, stirring occasionally, until the rice is tender but still holds a little bite in the center.
Add the blanched turnip greens and simmer for another 3 to 4 minutes until fully tender and the soup has a loose, spoonable consistency.
Remove the pot from the heat and stir in the remaining 20 g butter and the grated Casera cheese until melted and creamy.
Season with salt, pepper, and a pinch of nutmeg. Let the soup rest for 5 minutes off the heat before ladling into bowls.