Heat the oil in a large pot over medium heat. Add the mushrooms and cook until they have released their moisture, approximately 5 minutes. Remove the mushrooms from the pot and set aside.
Place the stock in a pot and heat over medium low. You want the stock to be warm when adding it to the risotto so that the temperature change does not affect the rice.
Add 2 Tbsp butter to the pot and let melt. Add the red onion and sage. Cook until the onions are translucent, approximately 3-5 minutes.
Add the rice and stir. Continue cooking to slightly toast the rice until it is just starting to show a little brown color, approximately 3-5 minutes.
Add the white wine to the pot. Cook until the rice is almost dry, where most of the wine has been absorbed.
Add 1/2 cup of stock to the rice. Stir continuously until most of the stock has been absorbed. Repeat this process one cup of stock at a time until the rice is cooked thoroughly. You want the rice to have just a little bit of bite left so that it is not completely mushy. Depending on your rice, you may need more or less stock for the right consistency so start tasting the rice after 4 cups have been added.
When your rice is cooked through, remove the pot from the heat.
Add in the turkey, green bean casserole, gravy, mushrooms, and Parmesan. Stir to combine.
Season to taste with salt and pepper as needed. Add additional turkey, green bean casserole, or gravy as desired to taste.