Chicken and Fennel Salad Sandwiches

2 cooked and diced or shredded chicken breasts
1 1/4 cups mayonnaise
1/2 cup plain yogurt
1 teaspoon finely grated fresh lemon zest
1/4 cup fresh lemon juice
1 1/2 teaspoons fennel seeds, toasted and cooled, then lightly crushed
1 1/2 teaspoons salt
1 teaspoon black pepper
1 cup chopped fresh basil
1 cup chopped fresh mint
3 cups chopped fennel bulb (sometimes called anise; from 2 to 3 medium bulbs, stalks discarded)
12 rolls or buns

Stir together mayonnaise, yogurt, zest, lemon juice, fennel seeds, salt, pepper, mint, and basil, then pour over chicken in bowl. Add chopped fennel and stir to combine. Season with salt and pepper.

Serve chicken salad in rolls.