Sauté onion and mushrooms in olive oil and butter over medium heat.
Add rice and white wine. Stir with a wooden spoon and cook over medium heat for 2 minutes. Season with salt and black pepper.
Add 1 cup chicken broth. Continue to cooking and stirring until liquid is absorbed. As the mixture cooks gradually stir in the remaining broth a little at a time. Let the rice absorb the chicken stock before adding more.
Cook time is 20-22 minutes. The rice will be just cooked and slightly chewy. Remove from the heat and stir in the mascarpone and shredded cheddar cheese. Stir to combine and melt the cheese. Serve immediately.