Heat oven to 350°F. Line 9-inch-square baking pan with foil, leaving overhang. Coat with cooking spray.
In bowl, combine flour, sugar, cinnamon, nutmeg. Cut in butter until crumbly. Toss in walnuts.
In large bowl, beat butter and sugar until smooth, 2 minutes. Beat in egg, vanilla and yogurt. Alternately beat in baking mix with milk. Scrape into prepared pan. Sprinkle with berries, then topping.
Bake in 350°F oven until toothpick tests clean, 50 minutes. Cool in pan on rack 10 minutes. Lift cake from pan. Let cool. Remove foil. Cut, serve.