Heat a large skillet over medium-high heat. Add 2 Tablespoons of butter and saute onion for 6-7 minutes or until it begins to soften. Add garlic cloves and cook for 1-2 minutes longer.
Add rice and cook for 2-3 minutes. Stir in 1 cup of chicken stock at a time. Let the rice absorb the chicken stock before adding another cup.
Once the rice has simmered for about 20-25 minutes and all of the chicken stock has been absorbed, stir in parmesan cheese. Season with salt and pepper.
While the risotto is cooking, heat another skillet over medium-high heat. Add remaining 4 Tablespoons of butter to skillet. Add mushrooms and saute until the mushrooms become golden and slightly crispy, if so desired. Sprinkle with salt. Serve on top of risotto.