Place potatoes in a large pot of salted water, bring to a boil, and simmer for 15 to 20 minutes until tender.
Coat chicken breasts evenly on both sides with the Cajun seasoning.
Heat olive oil in a large skillet over medium-high heat.
Sear the chicken for 6 to 8 minutes per side until the internal temperature reaches 165°F (74°C), then remove from the pan and let rest.
In the same skillet, add the bell peppers and onions; sauté for 5 to 7 minutes until softened and slightly charred.
Drain the potatoes and mash with butter and heavy cream until smooth; season with salt and pepper to taste.
Slice the chicken and serve alongside the mashed potatoes, topping the chicken with the sautéed pepper and onion mixture.