Add the white onion, garlic cloves, white vinegar, lemon juice, olive oil, chiles, smoked paprika, rosemary, cayenne pepper, salt, and 2 tablespoons of water to a blender.
Blend well for 20 seconds to create the peri-peri sauce.
Mix 1 teaspoon of the peri peri sauce into the mayonnaise and set aside.
Add the peri-peri sauce to the chicken thighs.
Make sure the chicken is throughly coated, and then leave the chicken to marinate for at least 30 minutes.
Heat a cast iron pan to a high heat.
Add the marinated chicken thighs to the pan.
Cook for 6 to 8 minutes on one side, until nicely charred.
Flip the chicken thighs and cook for another 6 to 8 minutes until cooked through, then remove from pan.
To assemble the sandwich, spread a spoonful of peri-peri mayonnaise on the base of the bread buns.
Top with arugula, sliced tomatoes, and red onion.
Top with two pieces of peri-peri chicken.
Top with more peri-peri mayonnaise and the lid of the bun, and serve immediately.