Preheat an oven to 375°F. Lightly oil an 8-inch springform pan. Dust the pan with flour and tap out the excess.
In a large bowl, whisk together the flour, almond meal, baking powder and salt. In another bowl, combine the oil, egg, half-and-half, lemon zest, lemon juice, the 1 cup sugar and the almond extract. Whisk to blend thoroughly. Add the oil-egg mixture to the flour mixture and whisk until just combined.
Pour the batter into the prepared pan. Arrange the plum halves, cut side up, on top of the batter. In a bowl, stir together the almonds, the 2 Tbs. sugar and the butter and mix well. Dot the almond topping over the cake.
Bake until the topping is golden brown and a cake tester inserted into the center of the cake comes out clean, about 45 minutes. Transfer the pan to a wire rack and let cool for 20 minutes. Remove the ring from the pan and place the cake on a serving platter. Cut into wedges and serve warm or at room temperature. Makes one 8-inch cake; serves 8.