Preheat the oven to 350°F (175°C). Butter a 9-inch round cake pan and line the bottom with parchment paper.
Pit and halve 500 g sweet cherries. Toss with 1 tablespoon fresh lemon juice and 2 teaspoons cornstarch mix until evenly coated. Set aside while you prepare the caramel.
Place 70 g unsalted butter in a small, light-colored saucepan over medium heat. Cook, swirling occasionally, until the butter foams and the milk solids turn golden brown and smell nutty, about 4-5 minutes. Watch closely near the end.
Stir in 70 g coconut sugar, 2 teaspoons redcurrant jam, the Zest of ½ lemon and ⅛ teaspoon fine sea salt. Cook for 1-2 minutes, stirring until the sugar dissolves and the mixture is smooth and glossy.
Pour the caramel immediately into the prepared pan, then tilt the pan to spread it into an even layer.
Arrange the cherries in a single layer, cut side down, in a decorative pattern over the caramel, starting at the outer edge and working toward the center. Pack them tightly.
In a large bowl, whisk together 1150 g all-purpose flour, 40 g light rye flour, 20 g finely ground almond flour, 1½ teaspoons baking powder, ½ teaspoon fine sea salt, and ¼ teaspoon ground cardamom. Set aside.
In a medium bowl, beat 115 g unsalted butter with 105 g coconut sugar until light and fluffy. Add 2 large eggs, one at a time, mixing well after each. Then mix in ½ teaspoon vanilla extract, ¼ teaspoon almond extract, the Zest of 1 lemon, and 120 g śmietana or full-fat sour cream.
Add the flour mixture and mix just until combined. Stop as soon as the flour disappears.
Carefully spoon the batter over the cherries. Use an offset spatula to gently smooth the top without disturbing the fruit underneath.
Bake for 45-55 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean. Rotate the pan halfway through baking.
Cool in the pan for 10-15 minutes. Run a thin knife around the edge. Set a serving plate on top and flip the cake in one confident motion. Leave the pan in place for 1-2 minutes before lifting it off slowly. If any cherries stick, press them back on while everything is still warm.
Notes
Don't overmix the batter. Once the dry ingredients go in, mix just until the flour disappears. Overmixing develops gluten and makes the cake dense instead.The 10-15 minute cooling window is real. Flip too soon, and the caramel runs everywhere. Wait too long, and it hardens and sticks to the pan. Set a timer if you need to.