Preheat your oven to 350 degrees Fahrenheit and spray a 9x5" loaf pan with cooking spray.
**Rinse the nectarines and dry completely. Slice as much of the flesh off of the nectarines as you can, discarding the pits and stems. Place the nectarine flesh, skin and all, into a high speed blender until completely smooth. Measure 369g (1½ cups) and pour into a stand mixer bowl fitted with a beater attachment.
To the bowl with the nectarine puree, add the yogurt, eggs, olive oil, stevia extract, vanilla extract, butter flavor, and lemon zest. Mix on low speed while you gather the dry ingredients.
In a medium-sized bowl, whisk together the whole wheat pastry flour, erythritol, starch, protein powder, baking powder, baking soda, and salt.
Turn off the mixer, dump in the dry ingredients, and returned to low speed. When it's about halfway mixed through, scrape down the sides of the bowl, then return to mixing. Mix JUST until combined, do not overbeat the batter.
Pour the batter into the prepared loaf pan and bake for ~55 minutes. The surface should have browned, and will spring back when tapped, and a toothpick should come out clean. Let cool in the pan, then slice and serve!