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1
Heat oven to 350u00b0 F. Grease with olive oil a 10-inch enameled or well-seasoned cast-iron skillet. (If your skillet isn't well-seasoned, line the bottom with a round of parchment paper.)
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2
To make crumble topping: in a small bowl, whisk together flours, brown sugar, cinnamon, salt, and almonds. Add olive oil, a little at a time, stirring to thoroughly and evenly moisten the crumbs. Aim for a mix of larger and smaller clumps. Put the bowl in the freezer while preparing the rest of the cake.
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3
In a large bowl, whisk together the dry ingredients: all-purpose flour, rye flour, light brown sugar, baking powder, and salt.
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4
In a separate bowl, whisk together the wet ingredients: olive oil, eggs, sour cream, lemon zest, and lemon juice.
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5
Pour the wet ingredients into the dry ingredients and fold with a spatula until just combined.
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6
Scrape the batter into the prepared skillet. Arrange plum wedges snugly and evenly over the batter, skin-sides up. Gently press them down into the batter; you should see their tops. Remove the crumble topping from the freezer, and scatter it evenly over the top. It won't completely cover the batter (by design; you want to see those juicy plums peeking through).
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7
Bake until golden brown and firm to the touch, about 40 to 45 minutes or until a paring knife inserted into the middle comes out clean. Start checking at about 35 minutes for doneness.
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8
Let cool in the skillet for about 30 minutes, and serve warm or at room temperature, straight from the pan.
