A nine-inch orange-and-grapefruit upside-down cornmeal cake cut into eight slices.

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Heat oven to 350°F. Line the bottom of a solid-bottom 9-inch cake pan with parchment, spread 3 tbsp melted butter and scatter 1/3 cup packed brown sugar evenly. Remove peel and white pith from one orange and one grapefruit, then slice into thin crosswise circles, remove seeds and arrange a single overlapping layer over sugar.
Whisk 1 cup flour, 1/2 cup fine cornmeal, 1 1/2 tsp baking powder and 1/2 tsp salt. Separately whisk 3/4 cup sugar, two eggs, 1/2 cup olive oil, 1/2 cup plain yogurt and 1 tsp orange zest. Fold dry into wet until just combined.
Spread batter gently over citrus. Bake approximately 45–50 minutes until the cake portion passes a skewer test without wet batter; fruit syrup on the skewer is not raw cake. Rest in pan about fifteen minutes.
Loosen sides, invert onto a serving plate while still warm and remove pan and parchment carefully. Cool roughly forty-five minutes more, then cut eight provisional slices. Verify citrus bitterness, topping release, moisture and timing in a kitchen trial.

Recipe-development draft; not kitchen-tested. Timing and yield are estimates. Nutrition has not been calculated.

Additional chilling, resting or cooling time: 60 minutes, included in the total.

Untested estimate: 45–50-minute bake, fifteen-minute pan rest and 45-minute cooling after inversion; preheat excluded.