Make ahead and store in the refrigerator: If you want to prep ahead to serve in a few days, you can prep up to the point of baking, then cover and refrigerate up to 4 days before baking. When you're ready to cook, just continue with the baking instructions as listed, but add additional cook time to compensate for the pasta being cold instead of hot. Store it in the freezer: If you want to prep this Turkey Tetrazzini ahead to serve later, you can prep up to the point of baking, then wrap tightly (I like to use a layer of plastic wrap and a layer of foil), and freeze up to 3 months. To serve, either thaw overnight in the fridge before baking, or pop in the oven from frozen before starting the oven. This way, as your oven heats gradually, it will warm your pan gradually so there is no temperature shock. To cook from frozen, you will need to add additional time -- it will likely require a bake time of 60 minutes to heat through.
- Place turkey breast in a large skillet and pour in turkey stock. Season turkey with salt and pepper.
- Bring to a simmer over medium heat and reduce to medium low. Cover and simmer until cooked through (165 degrees F inside) and tender, about 40 minutes.
- Alternatively, you can place the ingredients in a slow cooker and cook on low for 4 hours.
- Remove to a cutting board and chop or shred. Set aside.
- In a large skillet, melt butter.
- Add onion and saute over medium heat until onion is softened.
- Add garlic, Italian seasoning, salt, and pepper and cook 1 minute.
- Whisk in flour until completely absorbed by the butter.
- Add turkey stock and cream and whisk until smooth. Continue cooking over medium heat, whisking often, until thickened. Set aside.
- Preheat the oven to 400 degrees if baking immediately.
- Place cooked pasta, shredded turkey, and peas in a 9x13" greased baking dish. Pour sauce over top and spread with a spatula over pasta so that it soaks down into the spaghetti.
- Sprinkle with cheese and bake for 20 minutes or until golden and bubbly (see post above for make ahead instructions).