Make the sauce by melting the butter in a saucepan; stir in the flour until blended. Gradually stir in the milk. Crumble the cream cheese and stir until melted. Season with salt, pepper, and Worcestershire. Taste and see if you need to adjust seasoning.
While the sauce is cooking, make the pasta according to package directions; drain.
In a large bowl, combine the cooked pasta with the sauce; add the tuna, frozen peas, mushrooms, and pimentos. Mix well and spoon into a 9×13 prepared (greased) pan.
In a small bowl add the crushed Ritz craackers, melted butter, and Parmesan cheese. Mix well and sprinkle on top of the casserole.
Bake for 20-25 minutes or until crumbs are golden brown and casserole is bubbling around the sides.