Preheat the oven to 180 C (350F). Grease a 9x5 inch loaf pan with non stick spray, then line with parchment paper with some overhang so you can lift the finished cake out easily.
Whisk the flour, baking powder, salt and turmeric in a bowl.
In a large bowl, combine the one cup of sugar with the lemon zest, rubbing the lemon zest into the sugar with your fingertips until the sugar is moistened and pale yellow. Whisk in the sour cream or yogurt, eggs, and lemon juice until well combined.
Use a spatula and add the wet ingredients to the flour mixture, stirring until just combined. Fold in the melted butter. Pour the batter into the prepared loaf pan, smoothing out the top, then topping with lemon slices placed over the top. Sprinkle the tablespoon of sugar evenly over the top.
Bake until cake is golden brown and toothpick comes out clean (a few crumbs are ok), 50-60 minutes.