3/4 cup sugar, plus more to taste
2 Tbsp. cornstarch
1/4 tsp. ground cinnamon
1/2 cup cranberry juice cocktail or orange juice
1/2 tsp. freshly grated lemon zest
4 cups fresh, frozen (thawed; see Tip) or canned (drained) pitted sour cherries (see Sources)
1-3/4 cups blueberries, blackberries and/or chopped (unpeeled) purple plums
1 cup all-purpose flour
1/3 cup whole-wheat pastry flour (see Sources)
1-1/2 Tbsp. sugar
1-1/4 tsp. baking powder
1/2 tsp. salt
1/4 tsp. baking soda
2-1/2 Tbsp. unsalted butter, very cold, cut into bits
2 Tbsp. canola oil
3/4 cup nonfat buttermilk, plus more as needed
1-1/2 tsp. sugar mixed with 1/4 tsp. ground cinnamon, for garnish
1. To prepare fruit: Stir together 3/4 cup sugar, cornstarch and cinnamon in a 9- to 10-inch non-reactive deep-sided skillet or 3-quart wide-bottomed saucepan or Dutch oven (see Note). Stir in cranberry (or orange) juice and lemon zest, then the cherries and other fruit. Bring the mixture to a gentle simmer over medium heat, stirring. Simmer, stirring, until the mixture thickens slightly, about 2 minutes. Remove from the heat, taste and add up to 2 tablespoons more sugar if desired.
2. To prepare dough: Whisk all-purpose flour, whole-wheat flour, sugar, baking powder, salt and baking soda in a medium bowl. Add butter and oil. Using a pastry blender, two knives or a fork, cut in the butter until the mixture resembles coarse meal. Add 3/4 cup buttermilk, mixing with a fork just until incorporated. The dough should be very soft and slightly wet; if necessary, stir in a little more buttermilk. Let the dough stand for 3 to 4 minutes to firm up slightly.
3. To finish: Use lightly oiled soup spoons to scoop up the dough, dropping it in 8 portions onto the fruit, spacing them evenly over the surface. Return the slump to the stovetop and adjust the heat so it simmers very gently. Cover the pot tightly, and continue simmering until the dumplings are very puffy and cooked through, 17 to 20 minutes. Cut into the center dumpling with a paring knife to check for doneness. Let the slump cool on a wire rack, uncovered, for at least 15 minutes. Sprinkle the cinnamon sugar over the dumplings. Serve warm.