Pear-Walnut Upside-Down Cake

FOR THE TOPPING: Adjust oven rack to middle position and heat oven to 300 degrees. Grease 9-inch round cake pan and line bottom with parchment paper. Pour melted butter over bottom of pan and swirl to evenly coat. Combine sugar, cornstarch, and salt in small bowl and sprinkle evenly over melted butter.

Peel, halve, and core pears. Set aside 1 pear half and reserve for other use. Cut remaining 5 pear halves into 4 wedges each. Arrange pears in circular pattern around cake pan with tapered ends pointing inward. Arrange two smallest pear wedges in center.

FOR THE CAKE: Pulse walnuts, flour, salt, baking powder, and baking soda in food processor until walnuts are finely ground, 8 to 10 pulses. Transfer walnut mixture to bowl.

Process eggs and sugar in now-empty processor until very pale yellow, about 2 minutes. With processor running, add melted butter and oil in steady stream until incorporated. Add walnut mixture and pulse to combine, 4 to 5 pulses. Pour batter evenly over pears (some pear may show through; cake will bake up over fruit).

Bake until center of cake is set and bounces back when gently pressed and toothpick inserted in center comes out clean, 1 hour 10 minutes to 1 1/4 hours, rotating pan after 40 minutes. Let cake cool in pan on wire rack for 15 minutes. Carefully run paring knife or offset spatula around sides of pan. Invert cake onto wire rack set in rimmed baking sheet; discard parchment. Let cake cool for about 2 hours. Transfer to serving platter, cut into wedges, and serve.