Place nectarine slices in a bowl with lemon juice and sugar and steep for half an hour or up to three hours.

Place an oven rack in the lower third of the oven and preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius).

Grease and flour a 9x5x3-inch nonstick loaf pan.

In the bowl of an electric mixer fitted with the beater attachment, beat margarine or oil and sugar on high until the mixture becomes fluffy and turns a pale yellow color.

Meanwhile, combine the flour, baking powder, and salt in a bowl.

Add beaten margarine or butter, along with the ground vanilla beans or extract. Add one egg at a time, until incorporated.

Pour half the mixture into the prepared pan.

Drain the juice from the nectarines and then arrange over the batter in the pan. Cover the fruit with the remaining batter.

Bake until a cake tester inserted into the center of the cake comes out clean, about one hour.

Cool for 20 minutes.  Remove the cake from the pan and transfer to a wire rack to cool completely.

Serve with vanilla ice cream or garnish with non-dairy whipping cream and fresh nectarine slices, if desired.