In a bowl of stand mixer, whisk together 1/2 cup (115g) sourdough starter and 1 cup (240g) kefir or buttermilk.
Add 1 egg, 1/2 cup (100g) sugar, 1/2 teaspoon salt, zest of 1 lemon and 1 teaspoon vanilla extract. Mix well.
Add 2 cups (260g) sifted flour, and incorporate into the wet ingredients.
Connect the bowl to a stand mixer with a paddle attachment.
With the mixer running, add butter, 1-2 pieces at a time.
Scrape the bowl if needed, continue mixing until is batter is smooth.
Cover and leave at warm room temperature for 6-8 hours.
Line 9 x 13" sheet pan with parchment paper.
Cut each plum half into 5-6 pieces, and arrange in one layer on baking pan.
Sprinkle 2 tablespoons sugar over the plums.
Drop spoonfuls of batter on top of plums and spread evenly until all plums are covered.
Leave at warm room temperature for 2 hours (until batter softens and rises some).
Preheat oven to 350ºF (180ºC).
Bake plum pie for 30 minutes or until the top browns slightly. Turn pan once while baking.
Cool in the pan, then invert the pie onto a serving platter.
Slice into 18 pieces.