In a large frying pan, sauté onion, mushrooms, olive oil and minced garlic over medium heat until the onions start to look translucent and the mushrooms have just started to shrink in size.
Add the gluten-free flour to the frying pan and lightly toss to coat all the vegetables.
Add the heavy cream, chicken broth, salt and pepper to the frying pan and whisk well to combine. Turn the heat down to low-medium and bring to a light simmer. Let simmer for 5 minutes, stirring often, until the sauce starts to thicken.
Remove frying pan from heat then stir in 1 cup of the shredded cheddar cheese (set the remaining ½ cup of shredded cheddar aside for now) and Parmesan cheese until the cheese is completely melted.
Stir the drained tuna and frozen peas into the mixture until well mixed.
Place the al dente cooked pasta into a 9” x 13” baking dish.
Pour the sauce mixture over the pasta in the baking dish and stir well until all of the pasta is coated with sauce.
In a medium bowl, stir together the gluten-free breadcrumbs, remaining ½ cup of shredded cheddar cheese, melted butter and Italian seasoning together until mixed well.
Sprinkle the breadcrumb mixture evenly over the pasta mixture in the baking dish.
Bake the tuna casserole in the oven for 20 minutes or until the breadcrumbs start to look golden brown.
Serve hot.
Notes
Cook the pasta until al dente; otherwise, it'll become mushy when baked with the other ingredients.
Full-fat heavy cream is best for thickening the sauce.
To Store: Keep leftovers in an airtight container in the fridge for up to 4 days.
To Reheat: Use the microwave or oven at 350°F to reheat leftovers.