Put the tomatoes and cucumber in a bowl, add 1 tbsp olive oil, season and turn to coat.
Set a large frying pan with 2 tbsp olive oil over a medium-high heat. Coat the chicken fillets in the BBQ seasoning and fry for 10-12 minutes, turning halfway, until golden and cooked through. Transfer to a plate and keep warm.
Return the pan to the heat with the remaining oil. Cook the onion for 2-3 minutes, until slightly softened, then tip onto a plate. Lightly toast the buns.
To assemble, spread some ketchup on each bun base. Top with lettuce, chicken and onions and serve with the tomato salad.