Apple-Walnut Upside-Down Cake

To make apple topping: Preheat oven to 375 degrees F. Coat a 9-inch round cake pan with sides at least 2 inches high with cooking spray.

Combine 3/4 cup sugar and water in a heavy-bottomed saucepan. Bring to a simmer over low heat, stirring occasionally. Increase the heat to medium high and cook, without stirring, until the syrup turns a deep amber color, about 5 minutes. (Swirl the pan if the syrup is coloring unevenly.) Immediately pour the syrup into the prepared cake pan.

Toss apple slices with lemon juice in a bowl. Evenly press the apples into the warm caramel and set aside.

To make walnut cake: Combine walnuts, flour, baking powder and salt in a food processor; process until the walnuts are ground to a coarse meal.

Beat 2 egg whites in a mixing bowl with an electric mixer until soft peaks form. One tablespoon at a time, beat in 1/3 cup sugar, continuing to beat until the egg whites are stiff and glossy; set aside.

Beat 2 whole eggs with the remaining 1/3 cup sugar in a separate mixing bowl until thick and pale, about 5 minutes. Beat in vanilla.

Whisk one-fourth of the reserved beaten egg whites into the whole-egg mixture. Sprinkle half of the dry ingredients over the top and fold in gently using a rubber spatula. Fold in the remaining beaten whites, followed by the remaining dry ingredients.

Spread the batter over the fruit in the pan. Bake until the top springs back when lightly touched and a skewer inserted in the center comes out clean, 40 to 45 minutes. Let cool in the pan for 5 minutes, then invert onto a serving plate, setting any stray apples back in place. Serve warm or at room temperature.