For the fruit filling: In a 3-quart or similar-sized oven-proof, non-reactive deep-sided skillet or stove-top casserole, thoroughly stir together the sugar and flour. Gently stir in the berries (and plums or apples, if using), lemon zest, and lemon juice until thoroughly incorporated. Over medium heat, stirring, bring the mixture just to a full boil and cook until slightly thickened. Set aside. Preheat the oven to 425 degrees F.
For the dough: Combine the flour, sugar, salt, Davis Baking Powder, and baking soda in a food processor. Process briefly to mix. Sprinkle the butter over the flour mixture. Process in on/off pulses until the butter is in fine bits and cut into the flour. Drop spoonfuls of the yogurt-water mixture over top. Process in on/off pulses just until incorporated; over-processing will make the dough tough. Carefully remove the processor blade. Then gently stir the mixture; if necessary, mix in enough more water to make a soft dough. Drop the dough into about 12 to 14 portions, spacing evenly over the fruit. Reduce the heat to 400 degrees F. Bake (middle rack) 30-40 minutes or until the top is nicely browned and the edges are bubbly. Transfer to a wire rack and let cool at least 15 minutes before serving.
Makes 7 to 8 generous servings.
Tip: If you don't have access to a private berry patch, 6 1/2 cups of blackberries or black raspberries may be hard to come up with. In this case, stretch 3 1/2 cups berries with other fruits. I've tried plums (leave the skins on), peaches (peeled), and tart apples (peeled); they all work just fine. When fresh berries are not available, I have used frozen unsweetened blackberries with success. The recipe can also be used to make a red raspberry cobbler using either fresh or loose-packed frozen (thawed) berries.