In a large mixing bowl, cream together butter and sugar.
Add egg and continue to mix until combined.
In a smaller, separate bowl, whisk together buttermilk and sour cream.
In another bowl, whisk together flour, thyme, lemon peel, baking soda and salt.
With the mixer on low, add half of the flour mixture. Follow with half of the buttermilk mixture and continue to beat until combined. Repeat with remaining ingredients, scraping the side of the bowl as needed.
Pour batter into a greased 8-in. x 4-in. loaf pan.
Bake for 40-50 minutes or until a toothpick inserted near the center of the bread comes out clean. Remove from oven and allow to cool for 10 minutes before removing from the pan and transferring to a wire rack to continue cooling.