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 (9g) 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 rest for a few minutes before slicing.
In a pot, cook the onion over medium heat until softened, then add the garlic and 2 teaspoons (4g) of the Cajun spice mix. Cook for a further 30 seconds until fragrant. Add the basmati rice, cold water, and a pinch of salt. Bring to a boil, reduce the heat to low, cover, and cook for 12 minutes. Remove from the heat and let sit, covered, for 4 minutes before fluffing with a fork.
For the Cajun mayo, combine the mayonnaise with the remaining 2½ teaspoons of Cajun spice mix in a small bowl. Taste and adjust seasoning if needed — add a little extra salt or a splash of lemon juice if desired.
To assemble, serve the sliced Cajun chicken over the Cajun rice, topped with or served alongside the Cajun mayo.