Preheat oven to 350 degrees F (175 degrees C). Spread walnuts onto a baking sheet.

Toast walnuts in the oven until golden brown and fragrant, about 5 minutes. Set aside to cool.

Increase oven temperature to 375 degrees F (190 degrees C).

Place 3 tablespoons of flour, 3 tablespoons brown sugar, and 1/2 cup cooled walnuts in a bowl. Cut in 2 tablespoons butter until the mixture resembles coarse crumbs.

In another bowl, mix 2 cups flour, 2 tablespoons brown sugar, baking powder, and salt. Cut in 1/2 cup butter until mixture resembles fine crumbs. Stir in remaining 1/2 cup walnuts.

Whisk maple syrup and egg together; stir into the flour mixture. Gradually mix in milk to form a dough ball.

On a floured surface, knead dough lightly 10 times. Pat into an 8-inch circle on an ungreased baking sheet. Brush with milk and sprinkle topping evenly. Score into 8 wedges, but do not separate.