1. Combine peach slices, rhubarb, orange juice, orange zest and 1/4 cup of the honey in a large nonreactive bowl, toss to coat, and set aside while you make the batter, at least 5 minutes.
2. Heat the oven to 350°F and arrange a rack in the middle. Coat a (9- or 10-inch) springform pan with oil then line the bottom with a round of parchment. Whisk together the almond meal, polenta, flour, baking powder, and salt in a medium bowl. Set aside.
3. In the bowl of a stand-mixer fitted with the paddle (or using a handheld mixer), combine the sugars and eggs and beat on medium-low until just combined, about 30 seconds. Raise the speed to medium and beat until light colored and thoroughly combined and slightly thickened, about another 1 minute. Add the oil and vanilla and mix for an additional 1 minute until well combined and mixture resembles wet sand.
4. With the paddle still running, gradually add the dry mixture, about 1/2 cup at a time. Stop as soon as you no longer see any dry flour (the mixture should be thick like cornbread batter). Remove the bowl from the stand-mixer. Arrange fruit mixture on bottom of the cake pan (reserve excess juice for serving and do not add to the cake pan) then spoon the batter evenly over the fruit and spread it out evenly.
5. Bake until the cake is nicely browned and a cake tester or toothpick comes out with just a few crumbs on it, about 50 to 65 minutes (the 9-inch cake pan will take longer to bake). Run a knife along the edge of the cake then let it cool completely, at least 1 1/2 hours. Release the spring and invert the cake onto a serving platter or cake stand. Serve with a sprinkling of powdered sugar and the reserved fruit juices on the side.
6. Arrange fruit mixture on bottom of the cake pan (including any excess juices) then spoon the batter evenly over the fruit and spread it out evenly. Bake until the cake is golden brown, evenly risen, springs back when lightly pressed, and a cake tester or skewer inserted in the middle comes out with only a few crumbs, about 35 to 45 minutes.
7. Cool in pan on a wire rack 15 minutes. Place a plate upside down on top of cake pan; invert cake onto plate. Serve warm or at room temperature, preferably with ice cream or whipped cream.