Preheat the oven to 425 degrees. If you're making the biscuits from scratch, you can go ahead and make the dough now and set it aside until you're ready to bake the casserole.

In a dutch oven or other heavy bottomed pot, melt the butter and oil over medium heat. Add the onion and leeks and saute until starting to soften and brown, about five minutes. Add the chopped chicken and cook until browned on the outside. Stir in the mushrooms and garlic and cook for an additional 1-2 minutes. Sprinkle the entire mixture with salt, pepper, and flour. Add the chicken broth, using it to scrape up any browned bits on the bottom of the pan. Add the butternut squash and parsnips and bring the mixture to a boil. Cook, stirring often, until mixture is thickened. Reduce heat to low and continue to simmer an additional 5-10 minutes or until squash is tender. Remove from heat.

Pour the mixture into an 8 x 8 inch baking pan or other 2 quart casserole dish. Top with the biscuits, spacing them about 1/2 inch apart. then bake for 12-14 minutes or until biscuits are golden and cooked through.