Cook your mushrooms: Place a large pot over medium-high heat and add in 1 tablespoon butter. Once butter melts, add in the mushrooms, onions, thyme and salt and pepper. Stir occasionally until mushrooms and onions are cooked down about 4-6 minutes. Transfer to a bowl.
In the same skillet you cooked mushrooms, add in 2 tablespoons of butter and place over medium heat. Once butter is melted, whisk in a little bit of the flour and then slowly add in milk, a little bit at a time, alternating with the flour and vigorously whisking away any lumps. Bring mixture to a boil, then reduce heat and simmer for a few minutes stirring every so often, until the sauce thickens up. If it gets too thick, add in ¼ cup more milk.
Once thick, turn off the heat and stir in garlic powder, parmesan cheese. salt and LOTS of freshly ground black pepper.
Stir in cooked noodles, mushroom and onion mixture, drained tuna and peas. Season again with salt and pepper, to taste. Pour mixture into your prepared baking pan.
How to make dairy free tuna noodle casserole: If you want to make this tuna noodle casserole dairy free, I suggest using vegan butter, a dairy free milk of choice and dairy free parmesan cheese.
How to make gluten free tuna noodle casserole: This tuna noodle casserole can easily be made gluten free too! All you'll need to make sure to do is use gluten free pasta noodles, a gluten free all purpose flour and gluten free breadcrumbs (or substitute crushed up gluten free tortilla chips, potato chips or crackers).