Make the berry sauce: in a large pot combine 5 cups frozen mixed berries (or fresh), ½ cup water, and ½ cup pure maple syrup. Bring to a simmer over medium heat and cook, stirring occasionally, until the berries are heated through and the sauce is bubbly.
In a small bowl whisk together 2 tablespoons cornstarch and 2 tablespoons water to make a slurry. Slowly whisk the slurry into the simmering sauce and continue to cook, stirring, until the sauce thickens. Remove the pot from the heat and set the sauce aside to cool to serving temperature or refrigerate for later.
Preheat the oven to 350°F (175°C). Grease a 9-inch pie dish or round cake pan with non-stick cooking spray.
In a medium bowl whisk together the dry ingredients: 1½ cups all-purpose flour, 1 teaspoon baking powder, and ½ teaspoon baking soda.
In a separate large bowl whisk the wet ingredients: 1 large egg, 1 cup milk, 1 teaspoon vanilla extract, the juice of 1 lemon, and the zest of 2 lemons.
Add the dry ingredients to the wet ingredients and whisk until just combined and no large lumps remain. Do not overmix.
Pour the batter into the prepared pan and smooth the top with a spatula.
Bake for 23–25 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and let the baked pancake rest for a few minutes. Slice and serve warm, spooning the berry sauce over the slices.