Cook rice according to package directions.
Cook green beans according to package directions.
Drain; set aside.
Combine cream of celery soup, milk, pimiento, thyme, and cayenne.
Stir till smooth.
Stir half of the soup mixture into cooked rice.
Fold tuna into rice mixture.
Turn into a 1 1/2-quart casserole.
Spread cooked and drained green beans over rice.
Top with shrimp.
Pour remaining soup mixture over all.
Bake, covered, at 325° till heated through, 25 to 30 minutes.
Uncover and sprinkle with onions.
Bake till onions are crisp, about 5 minutes longer.