In a large sauce pan, heat 1/4 cup of butter over medium heat, add onion and saute for 5 minutes. Add garlic and cook for one more minute. Add heavy cream and bring to a simmer, lower the heat and add the gluten free linguine, turkey, peas & carrots, and 1 cup of Parmesan cheese.; stir to combine. Pour mixture into a greased 2 quart casserole dish. Melt remaining 1/4 cup of butter in a microwave safe dish, mix in 1 cup Parmesan cheese and panko. Sprinkle bread crumb mixture over the top of the casserole. Bake at 400°F for 30 minutes.