Spray a medium sized casserole dish, I used an 11x9, with nonstick spray. Set aside. While the crackers are in the oven, add the sliced mushrooms, onions, 2 tbsp butter and thyme to a skillet over medium heat and sauté until softened and mushrooms have shrunk. About 4 minutes.
In a large bowl, combine cooked noodles, soup, milk, peas, tuna, salt and pepper. Stir. Then fold in your sautéed mushrooms and onions. Pour into a baking dish and bake for 20 minutes.
While that's baking, crush your crackers. Put in a ziplock bag and roll with a rolling pin. Then toss with the remaining 1 tbsp melted butter.
After 20 minutes, remove casserole from oven. Give it a stir and top with the breadcrumbs. Put back in oven for 5-7 minutes until breadcrumbs are golden brown and crispy.