Cook corkscrew pasta according to package directions. Drain, cover. Set aside.
Preheat oven to 350 degrees.Spray 2 quart casserole dish with nonstick cooking spray;set aside.
Melt butter in large nonstick skillet over medium-high heat. Add mushrooms and onion;cook and stir until onion is tender, about 5 minutes.
Using wire whisk, stir together chicken broth, milk and flour in small bowl until well blended. Stir into mushroom mixture;bring to a boil. Cook and stir 2 minutes or until thickened. Reduce heat to medium.
Stir in tuna, peas, pimientos and thyme. Add pasta to tuna mixture and stir.
Spread pasta mixture evenly in prepared casserole. Bake until bubbly and heated through, about 30 to 35 mnutes. Let stand 5 minutes before serving.