If using uncooked chicken breasts, cook chicken in well-salted boiling water just until cooked through and center is no longer pink, 8-10 minutes depending on thickness. Cool and dice chicken. Otherwise, dice about 2/3 of the meat from a rotisserie chicken. Set aside.
In a bowl, stir together the celery, red bell pepper, red onion, parsley, dill, lemon juice, lemon zest, mayo, yogurt, salt, and pepper. Add in the diced/cooked chicken and stir gently until well combined.
Spread mayo on desired amount of bread slices, top with chicken salad, and layer on lettuce or spinach if desired. Cover with second slice of bread. Serve whole or cut into halves or fourths for tea-size sandwiches.
Leftover Lemon Chicken Salad can be wrapped airtight and chilled for several days.