In a small bowl or jar, combine all sourdough ingredients until smooth. Cover with plastic wrap and let sit at room temperature 12-18 hours or preferably overnight.
Biscuits
In a large bowl, combine flour, baking powder, baking soda, salt, and agave. Cut in butter with a pastry cutter or your fingers until butter is in pea-sized clumps. Add buttermilk and sourdough starter then gently stir until a shaggy ball of dough forms.
Turn dough onto a lightly-floured surface and gently knead several times until dough forms a smooth ball. Roll dough out to 1-in thickness and cut out biscuits using a 2-in biscuit cutter. Place biscuits 1-in apart upside-down on prepared baking sheet. Bake biscuits at 425F 11-12 minutes until golden brown.
Peach Preserves
In a small saucepan, bring peaches, sugar, and lemon juice to a simmer over medium-high heat. Reduce heat to medium-low, cover, and stir occasionally until peaches are reduced and syrupy, about 15 minutes.
Serve biscuits warm with peach preserves and enjoy!