To make the Cajun spice mix, combine the onion powder, garlic powder, sweet paprika, dried oregano, dried thyme, chilli powder, and a generous pinch of salt in a small bowl. Mix well and set aside.
For the Cajun chicken, halve the chicken breasts horizontally to create thinner fillets. Coat them with the avocado oil and rub in 3 teaspoons of the prepared Cajun spice mix. Heat a large pan over medium-high heat and cook the chicken for about 4 minutes per side until golden brown and fully cooked. Remove from the pan and let them rest for a few minutes before slicing.
In a pot, cook the onion over medium heat until softened, then add the garlic and Cajun spice mix. Cook for another 30 seconds until fragrant. Add basmati rice, cold water and a pinch of salt. Bring it to a boil, reduce the heat to low, cover, and cook for 12 minutes. Remove from the heat and allow it to sit, covered, for 4 minutes before fluffing with a fork.
Combine the mayonnaise with the remaining Cajun spice mix for the Cajun mayo in a small bowl. Taste and adjust seasoning if needed—add a bit of salt or a splash of lemon juice if desired.
To assemble the dish, serve the sliced Cajun chicken over the Cajun rice and top with or serve alongside the Cajun mayo.