Rub some oil over the bottom of a large cast iron skillet. (You can also use a baking dish wide enough to fit salmon and veggies.)
Slice peppers, onion, and garlic and mix it with Cajun seasoning and half of the olive oil.
Rub salmon pieces with remaining olive oil and then Cajun seasoning all over.
*Note: make sure your olive oil is good quality and has no bitterness to it or it will transfer into your dish. Olive oil can be substituted with canola oil if needed.*Tip: taste your Cajun seasoning mix before using it to check if more salt is needed. If needed, add a little salt to salmon and veggies.
Place salmon skin down in the skillet and spread veggies all around it.
Bake for 20-25 minutes, depending on the thickness of salmon.
You can check salmon by touch or use a meat thermometer. It is recommended by USDA to cook salmon to 145° internal temperature, which is when salmon is considered fully done. Personally, I prefer salmon at either medium or medium-well temperature, which is around 125°-130°.