Preheat oven to 350°F
Line a 9 x 5 inch loaf pan with parchment, or grease the pan well.
In a medium sized bowl, whisk together flour, baking powder, baking soda and salt. Set aside.
Add sugar and lemon zest to a large bowl, and using a mixer, beat on low for 2 to 3 minutes. (this infuses the sugar with lemon for a stronger taste)
Beat butter and oil in to the sugar and zest.
Add eggs, one at a time, beating after each addition.
Stir in lemon juice and sour cream.
Mix in flour mixture until smooth and creamy.
Pour half the batter into prepared pan.
Top with ½ cup of the lemon curd
Pour remaining batter over the lemon curd
Spoon the ¼ cup lemon curd on top of the batter and using a knife, swirl the lemon curd over the top of the loaf in any pattern you would like. ( I use an S pattern)
Bake in preheated oven for 40-50 minutes. If the loaf starts to brown too much, cover with foil for last 10- 15 minutes.
Allow to cool in the pan for 10-15 minutes
Remove from pan and finish cooling for 30 to 40 minutes before cutting.
Store leftover cake in the fridge in an air-tight container or wrapped in plastic wrap.