Bring 4 cups of water to a boil in a large saucepan. Turn off the heat, add the tea bags and let steep for 5 minutes.
Remove the tea bags and bring back to a rolling boil. Stir in 1/2 teaspoon salt and the chicken. Cover, reduce the heat to low and poach until fully cooked, about 7 minutes. Drain and transfer the chicken to a small bowl to cool to room temperature.
Stir together the onions, mayonnaise, yoghurt, basil, tarragon, vinegar and 1/2 teaspoon salt in a large bowl. Stir in the cooled chicken, cover and refrigerate for at least 1 hour to allow the flavors to combine.
Toast the almonds in a dry skillet over medium heat, stirring constantly, until golden, 3 to 4 minutes.
Divide the chicken among 4 of the bread slices, and top with the almonds, salad greens and the remaining bread slices. Cut in half and serve.