While the chicken is cooking, cut the burger buns in half and lightly toast them on the grill or in a toaster.
Once the chicken breasts are cooked, you can optionally add a slice of cheese on top of each breast and let it melt for a minute.
To assemble the burger, spread mayonnaise on the bottom half of each bun, followed by ketchup and mustard to taste.
Place a lettuce leaf on top of the sauces, then add the grilled chicken breast.
Top with tomato slices and red onion rings.
Finish by placing the top half of the bun on the assembled ingredients.