To make the topping, in a small bowl, add crackers crumbs and melted butter and toss to coat. Set aside.
To make the casserole, preheat oven to 450 degrees. Coat a 9-inch by 13-inch baking dish with nonstick spray.
Bring 4 quarts water and 1 tablespoon salt to boil. Add egg noodles and cook until al dente, about 7-8 minutes. Drain well.
Meanwhile, In a 12-inch skillet over medium-high heat, melt butter until foaming. Add mushrooms, onion, and 1/2 teaspoon salt. Cook until mushrooms are browned and their liquid has evaporated, about 15 minutes.
Add peas, tuna, parsley, broth, and cream. Bring to simmer, about 10 minutes. Add cooked noodles and toss to combine. Season to taste with salt and pepper (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Do not skip this step or your casserole may be bland!
Pour mixture into prepared baking dish. Sprinkle with bread crumb mixture. Bake until the sauce is bubbling and the crumbs are golden brown, about 10 to 15 minutes. Serve with lemon wedges.