Preheat oven to 425 degrees F. Whisk all cajun seasoning ingredients together in a small bowl or toss together well in a small resealable bag.
Place chicken skin side down on a large rimmed baking sheet and drizzle and rub 1 Tbsp of the olive oil evenly over thighs. Sprinkle with cajun seasoning to taste (I did 1 Tbsp total).
Rotate to opposite side and drizzle and rub tops with another 1 Tbsp olive oil. Season with cajun seasoning to taste (I did 1/2 Tbsp).
Toss potatoes with 2 Tbsp olive oil (I just did this on the cutting board I used to cut the potatoes) and season with cajun seasoning to taste (I did 2 tsp) and toss.
Transfer potatoes to baking sheet placing them around chicken thighs. Roast in preheated oven 30 minutes.
Remove from oven and add bell pepper and red onion over potatoes and toss, then return to oven and roast about 10 - 15 minutes longer until chicken is golden and cooked through and potatoes are tender (you can broil during last few minutes to help brown a little more if desired).
Serve warm sprinkled with parsley and season with salt and pepper to taste as desired.
If you have one, I recommend using a dark non-stick pan for one pan chicken and veggies recipes. I don't recommend them for baking, but when roasting veggies they can help brown them up better than an uncoated pan.