Heat a wide skillet or Dutch oven over medium heat. Add a tablespoon of olive oil and a tablespoon of butter.
Season the diced chicken with a pinch of salt and pepper. Sear until lightly browned and cooked through, about 5–7 minutes. Remove and set aside.
In the same pot, add the remaining butter. Sauté the onion until translucent, about 2–3 minutes. Add the garlic and mushrooms; cook until the mushrooms release their moisture and start to brown.
Pour in the Arborio rice. Stir for 1–2 minutes to toast the grains lightly, coating them in the fat and onions.
If using wine, add it now. Let it simmer gently until it has mostly evaporated, about 1–2 minutes.
Reduce heat to medium-low. Ladle in a small amount of warm stock and stir until absorbed. Continue adding stock, 1/2 cup at a time, stirring frequently and letting the rice release its starch.
After about 10 minutes, return the chicken to the pot and mix with the rice. Keep stirring and adding stock until the rice is creamy and al dente, about 18–20 minutes total.
Stir in the grated parmesan and a little extra salt and pepper to taste. If the risotto seems too thick, loosen with a splash of stock or water.
Finish with chopped parsley and a final pat of butter for shine. Serve hot, with extra parmesan on the side if you like.