In a large skillet heat the butter and the olive oil over medium high heat. Add the sliced onion and stir. Reduce heat to medium and sauté for 25-35 minutes until softened and caramelized, making sure to seasoning with salt and pepper as you go. Add the garlic and the leaves of 2 sprigs of thyme with 5 minutes to go and stir to combine. Remove ¾ of the mixture from the skillet and set aside.
In the same large skillet with the remaining onions and leftover oil/butter, add the rice to the skillet, and stir for 1 minute to just toast the rice.
Once the rice has slightly toasted, add the white wine and stir to combine. Wait until all the wine has been absorbed and then start adding the chicken stock 1/2 cup at a time, making sure to let each batch of chicken stock absorb before adding the next. Continue to stir the risotto the entire time, about 25-30 minutes. Once you've added all your stock, and the rice is tender, reduce the heat to low and stir in the Parmesan, mascarpone cheese and caramelized onions. Season with salt and pepper and extra thyme as needed. Serve immediately.