Preheat oven to 350 degrees F. Grease and line a 9-inch cake pan with parchment paper.
Make the oat pecan crumble. Melt the butter and let cool slightly. Combine the butter with the oats, brown sugar, pecans, and flour until well incorporated. Set aside.
Prepare the cake. Slice the peaches into ½ inch thick slices and set aside.
In a medium bowl, combine the flour, baking powder, and salt.
Cream together the softened butter with sugar in the bowl of a stand mixer until light and fluffy, about 2-3 minutes. Add the eggs one at a time, fully incorporating the egg before adding another. Add the vanilla kefir and vanilla extract, stirring to combine.
Add the dry ingredients, stirring just until combined. Pour the batter into the prepared cake pan and arrange the peach slices on top. Sprinkle the oat pecan crumble on top. Bake the cake for 45 -50 minutes or until a toothpick inserted in the center comes out clean. Remove from the oven and let cool completely.
Sprinkle with powdered sugar and serve at room temperature.