Bring the stock to a simmer in a saucepan. Partially cover and keep warm.
Warm the butter and olive oil in a large, heavy-bottomed saucepan over medium-low heat.
Add the onion and cook about 1 minute, until softened but not browned. Add the garlic and rosemary and cook for 30 seconds.
Add the rice and stir until the rice looks slightly opaque, 2 to 3 minutes. Add 1 tsp salt and the red wine and cook until the wine is absorbed.
Add enough stock to cover the grains and cook, stirring, until it is absorbed. Continue adding enough stock to cover the grains, stirring after each addition, until the rice is creamy and slightly al dente, about 18 minutes.
Stir in the fontina and arugula and remove the pan from the heat. Season with salt and pepper and server.