Put on rubber gloves before touching raw nettles. Wash them in a bowl of cold water and pick off any tough stems, keeping mostly leaves.
In a 24 cm / 9.5-inch heavy-bottomed pot, melt half the butter with the olive oil over medium heat. Add onion and garlic, cook 5 minutes until soft and translucent, not browned.
Add the diced potatoes and stir 2 minutes to coat them in the fat.
Pour in the hot broth, bring to a boil, then reduce to a simmer. Cover partially and cook 10 minutes, until the potatoes start to soften.
Stir in the rice. Simmer uncovered for 12 to 14 minutes, stirring occasionally, until the rice is nearly tender.
Add the nettles, still wearing gloves if handling them directly, and stir until wilted and deep green, about 4 to 5 minutes. The stinging hairs are neutralized within 2 minutes of boiling.
Season with salt and pepper. The soup should coat the back of a spoon lightly; loosen with a splash of hot broth if it's too thick.
Remove the pot from heat and stir in the remaining butter and half the grated Casera until melted and glossy.
Ladle into bowls, top with the remaining cheese and a drizzle of olive oil. Serve immediately while the nettles are still vivid green.