Crush Triscuit Crackers and 1/4 cup of walnuts together. Add 2 Tbsp. butter and flour. Mix well and press into 9" pie pan. Bake 8 minutes at 350 degrees F.
Saute celery, onion and garlic together until translucent. Add yams, water, half the salt and pepper and the allspice. Cover and simmer until yams are just fork tender. Remove from heat and mash roughly. Cool slightly. Beat eggs with half-and-half and remainder of the salt and pepper. Add yam mixture, slowly tempering the eggs. Pour into prepared shell, top with remaining walnuts.
Bake at 350 degrees F for approx. 35 minutes or until center is firm.