To make tzatziki sauce: Sprinkle cucumber with 1/4 teaspoon salt and leave in sieve to drain for about 30 minutes. Squeeze out water from cucumber with your hands but do not rinse. Mix cucumber with yogurt, 1 grated garlic clove, dill, vinegar and 1 to 2 tablespoons olive oil (start with less and add as needed). Season with salt and pepper. Refrigerate until ready to serve.
To prepare salad: Cook grains in chicken stock with a little salt, according to package directions. Drain and set aside.
Meanwhile, heat 4 tablespoons oil in large frying pan and cook onion over medium heat until softened, about 7 to 10 minutes. Add 3 chopped garlic cloves and red pepper flakes or chile, and continue to cook for a minute. Add cooked grains, chicken, oregano, lemon zest and juice, and stir to combine. Taste and adjust seasoning and balance as necessary with salt, pepper and more lemon. Mix in red or green pepper and parsley.
Transfer to serving dish. Cool and refrigerate (can be kept for a day). To serve, bring salad to room temperature, unless using rice, which can be kept chilled. Top with feta and watercress. Serve with tzatziki and lemon.