Put a large pot of water to boil. Add 1 teaspoon light oil to the water.
In a skillet, add a tablespoon of oil and heat 1 minute. Add the onion and garlic. Sauté for 5 minutes. Add in the mushrooms and cook on medium heat until softened.
Stir in the tapioca starch into the vegetables. This will prevent lumps when you add the liquids.
Add the rest of the liquid ingredients and half the cheese and mix. Add the rotisserie chicken and mix well. Bring to a boil.
Cook the spaghetti, but cook it for 1 minute less than the package directions. This will help the spaghetti hold up to being baked in the sauce better.
Turn the heat on the chicken mixture down to low heat and simmer for 5 minutes.
Drain the spaghetti in a colander and pour it back into the pot. Pour the chicken mixture over the pasta and mix it well.
Pour the tetrazzini into a casserole dish and top with the remaining shredded cheese. Bake at 375º F for 20 minutes until the cheese is bubbly. Serve hot.
I love to use leftover rotisserie chicken in this recipe. You can use any leftover chicken, but if you use boiled chicken, you will need to adjust the seasonings in the casserole.
Tapioca starch is just one gluten free option as a thickener. You can also use cornstarch, potato starch, or arrowroot starch.
The two brands of gluten free spaghetti I tested in this recipe were Jovial GF and Tinkyada. Both held up well to being baked in the sauce but I highly recommend cooking the pasta one minute less than the package directions.
You can use any cheese depending on the flavor you like for this chicken tetrazzini. I have used cheddar, pepper jack, and a Mexican cheese blend.
Many ask what brands of rotisserie chicken are gluten free. My first choice is Costco because their rotisserie chickens are gluten free. Nob Hill (part of Raley's grocery chain) and Safeway (part of Albertson's grocery chain) do not use gluten in their rotisserie chickens, but it is always best to ask in the store in case the suppliers they use are different.