Basil Chicken Salad with Mushrooms, Walnuts and Avocado on a Whole Grain Bread

chicken breast halves, about 1 1/2 lb. total, cooked (roasted)

fresh basil leaves, chopped

scallions chopped

small red onion, diced (more for serving, optional)

large avocado, firm but ripe, pitted, peeled and diced (more for serving, optional)

mushrooms of your choice

butter

walnuts, toasted and chopped

mayonnaise 

juice of 1/2 lemon

salt and freshly ground black pepper

slices of Whole Grain Bread , toasted