Make the guacamole: in a medium bowl, mash the chopped avocado with the lime juice and 1/4 teaspoon salt until mostly smooth but still slightly chunky.
Stir the minced 1/4 portion of the red onion and the minced 1/2 tomato into the guacamole; taste and adjust salt as needed. Set aside.
Heat a griddle or grill pan over medium-high heat. Grill the filleted chicken breasts for about 6–10 minutes per side, until cooked through and internal temperature reaches 165°F (74°C).
Let the chicken rest a few minutes, then slice thinly.
Warm the hard taco shells according to package instructions if desired.
Assemble tacos: place salad leaves in each shell, add a spoonful of guacamole, top with sliced chicken, then finish with the remaining sliced tomatoes and sliced red onion.