Topping:

1 1/2 c. flour
1/2 c. sugar
1 tsp. grated lemon peel
1 tsp. cinnamon
10 tbsp. (1 1/4 sticks) chilled unsalted butter, cut up
1 c. chopped walnuts

Filling:

8 c. fresh blueberries
3/4 c. sugar
2 tbsp. fresh lemon juice
1 1/2 tbsp. cornstarch
1 1/2 tsp. ground cinnamon

For Topping: Whisk first 4 ingredients in bowl to blend. Add butter and rub in with fingertips, pressing until moist clumps form. Mix in nuts (topping can be made 1 day ahead). Cover and chill.

For Filling: Preheat oven to 375°F. Place berries in 13 x 9 x 2-inch glass baking dish or 3-quart round baking dish with 2-inch high sides. Add sugar, lemon juice, cornstarch and cinnamon to berries. Toss gently to blend. Spread berry mixture evenly in dish; sprinkle topping over.

Bake until brown and filling is bubbling thickly, about 50 minutes. Cool.