In a large saute' pan; heat butter over medium-high heat until melted. Add onions & garlic. Saute 3-5 minutes or until wilted. Add shrimp; cook 7-10 minutes until pink.Add Rotel, cream of mushroom soup, & crab boil. Mixing well. Bring to boil. Lower heat and simmer 7-10 minutes.
Season with salt & pepper to taste if desired.
In a large mixing bowl, combine shrimp & sauce with the cooked yellow rice. Mix together well.
Pour into a greased 9x13 inch casserole dish. Cover with foil. Cook 20 minutes. Uncover; cook 5 more minutes.