Preheat oven to 350°.
Combine first 4 ingredients in a medium bowl; toss gently. Coat a 9-inch cake pan with cooking spray. Brush pan with 1 tablespoon melted butter. Sprinkle pan evenly with 1 tablespoon sugar. Pour nectarine mixture into pan in an even layer; set aside.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, cornmeal, baking powder, and salt in a bowl, stirring well with a whisk.
Combine remaining 1 cup sugar and 5 tablespoons softened butter in a large bowl; beat with a mixer at medium speed until well combined. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Add flour mixture to butter mixture; beat just until combined.
Spoon batter evenly over fruit mixture. Bake at 350° for 40 minutes or until cake is set and light brown. Cool in pan on a wire rack 5 minutes. Place a plate upside down on top of cake pan; invert cake onto plate. Serve warm or at room temperature.