1. In between 2 large sheets of plastic wrap, pound out 1 breast with the flat side of a meat mallet, until it becomes a thin piece, about 1/4- to 1/2-inch thick. Repeat with the rest of the chicken.
2.Heat a grill pan or preheat an outdoor grill to medium-high. Brush chicken breasts lightly with olive oil and season with coriander, salt, and black pepper to taste. Grill chicken, in batches if necessary, turning once, until cooked through, about 2 minutes per side.
3. Toss cucumber, tomatoes, onion, tarragon, lemon zest and juice, salt, and cayenne pepper in a serving bowl. Halve and pit avocado; press a knife into pit, twist, and lift out. Score flesh using a knife, and use a spoon to scoop it from skins and into salad; toss gently to combine. Serve chicken topped with salad.
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