Cook rice according to package directions.
Cook beans according to package directions; drain and set aside.
Mix soup, milk, pimiento, thyme, and cayenne till smooth.
Stir half of soup mixture into rice; fold in tuna.
Turn into a 1 1/2quart casserole.
Spread beans over rice; top with shrimp.
Pour remaining soup mixture over all.
Bake, covered, at 325° for 25 to 30 minutes.
Sprinkle onions atop; bake, uncovered, till crisp, about 5 minutes.