Prepare Cajun Seasoning Mix and Red Onion Salsa; set aside.
Rinse red snapper and pat dry with paper towels.
Sprinkle with Cajun Seasoning Mix.
Heat large, heavy skillet over high heat until very hot.
Add butter and swirl skillet to coat bottom.
When butter no longer bubbles, place fish in pan.
Cook fish 6 to 8 minutes or until surface is very brown and fish flakes easily when tested with fork, turning halfway through cooking.