Preheat oven to 350 degrees. Generously butter a 9-by-5-inch loaf pan. Dust with sugar and flour, and tap out excess; set aside. In a mixing bowl, cream butter until smooth. Add 1 cup sugar; beat until light and fluffy.
In a bowl, whisk together flour, cornmeal, lemon zest, baking powder, baking soda, and salt. In another bowl, whisk together eggs, buttermilk, and lemon juice.
With mixer on low speed, add flour mixture to butter mixture in two batches, alternating with the buttermilk mixture. Beat until just combined, scraping bowl as needed. Pour batter into prepared pan.
Bake until center is springy to the touch, about 55 minutes; drape with foil if cake browns too quickly. Let cool 10 minutes in pan; invert cake onto a wire rack. Reinvert and let cool completely.
In a skillet, stir together nectarines and remaining 2 tablespoons sugar; add 2 tablespoons water. Cook, stirring, until fruit is soft and liquid syrupy, about 5 minutes. Let cool before serving with the cake.