In a large skillet over medium high heat, melt the butter. Add the sliced mushrooms and sauté for 7-9 minutes until they are soft and starting to caramelize. Season with salt and transfer to a clean plate.
In the same skillet heat the olive oil over medium high heat. Add the shallot and sauté until translucent. Add the garlic and sauté for about 1 minute. Add the Arborio rice to the skillet, making sure to stir it around in the olive oil so that each granule is coated with it, and toast it for about 90 seconds.
Add the white wine and stir the rice until all the wine has been absorbed and then start adding the chicken stock 1/2 cup at a time, making sure not to add more until the previous addition has been absorbed.
Once the rice is just tender, add the Parmesan and mascarpone cheese and combine. Add the previously sautéed mushrooms and carefully combine into the risotto. Remove from heat and season with salt and pepper. Garnish with chives and a squeeze or two of lemon juice and serve.