Pistachio Lemon Shortbread

Preheat oven to 325°F.
Butter 13x9x2-inch metal baking pan. Combine all purpose flour, sugar, and semolina flour in processor; blend 5 seconds. Add butter, lemon peel, and vanilla. Using on/off turns, blend until coarse meal forms. Turn dough out into bowl. Add nuts and knead gently to combine. Press dough evenly over bottom of prepared pan. Using fork, pierce dough all over.
Bake shortbread until pale brown in center and golden at edges, about 35 minutes. Cool in pan on rack 10 minutes. Cut lengthwise into 4 strips, then cut each strip crosswise into 6 squares. Cool completely in pan. (Can be made 1 day ahead. Cover tightly with foil; store at room temperature.)

*Semolina flour, often labeled "pasta flour," is available at specialty foods stores, Italian markets, and some supermarkets