1. Preheat oven to 350º. Line a baking sheet with parchment paper. (This helps with cleanup.) Toss peaches with lemon juice. Sift together flour, baking powder, and baking soda.
2. Cook 1/2 cup granulated sugar in a 10-inch cast-iron skillet over medium heat, stirring occasionally with a wooden spoon, 10 minutes or until sugar melts and turns a deep amber color. Remove from heat. Immediately add 1/4 cup butter, stirring vigorously. Spread caramelized sugar to coat bottom of skillet evenly, and sprinkle with brown sugar. Arrange peach wedges in concentric circles over sugar mixture, overlapping as needed.
3. Split vanilla bean lengthwise, and scrape out seeds into bowl of a heavy-duty electric stand mixer. Beat vanilla seeds and remaining 3/4 cup granulated sugar and 1/2 cup butter at medium speed until smooth. Add eggs, 1 at a time, beating until blended after each addition. Add sour cream, beating until blended. Gradually add sifted flour mixture, beating at low speed just until blended and stopping to scrape bowl as needed. Spoon batter over peaches in skillet, and spread to cover. Place skillet on prepared baking sheet.
4. Bake at 350° for 40 to 45 minutes or until golden brown and a wooden pick inserted in center comes out clean. Cool in skillet on a wire rack 10 minutes. Run a knife around edge to loosen.
5. Carefully pour out any excess liquid from skillet into a measuring cup, and reserve. (It's okay if you don't have any excess liquid--it all depends on how juicy your fruit is.) Carefully invert cake onto a serving plate, and drizzle with any reserved liquid. Cool slightly (about 10 minutes). Cut cake into wedges using a serrated knife. Top with sweetened whipped cream, if desired, and serve immediately.