Prep the pan and oven. Heat the oven to 325°F (163°C).
Line the bottom of an 8- or 9-inch springform pan with parchment and lightly spray the sides.
Make the crust. In a bowl, mix almond flour, oats, brown sugar, melted light butter, vanilla, and salt. Stir until it feels like damp sand.
Press and pre-bake. Press the crust mixture firmly into the pan in an even layer. Bake for 8–10 minutes until set and lightly golden.
Let it cool while you make the filling.
Simmer the blackberry sauce. In a small saucepan, add blackberries, water, sweetener, and lemon juice. Cook over medium heat for 5–7 minutes, gently mashing, until juicy and slightly thick. If you like it smooth, push through a sieve.
Cool to lukewarm.
Blend the filling base. In a large bowl, beat softened cream cheese until smooth, about 1 minute. Add Greek yogurt, sugar or sweetener, lemon zest, lemon juice, vanilla, cornstarch, and salt. Mix just until smooth and no lumps remain.
Add eggs gently. Beat in the eggs one at a time on low speed.
Mix only until incorporated to avoid adding too much air.
Assemble. Pour the filling over the cooled crust and smooth the top. Spoon small pools of blackberry sauce over the surface. Use a skewer or knife to swirl lightly.
Don’t overmix the pattern.
Bake. Place the pan on the middle rack. Bake for 35–45 minutes, until the edges are set and the center still has a slight wobble. Avoid opening the oven often.
Cool gradually. Turn off the oven, crack the door open, and let the cheesecake rest inside for 30 minutes.
Then move to a rack and cool to room temperature.
Chill. Cover and refrigerate for at least 4 hours, preferably overnight. This sets the texture and deepens the flavor.
Serve. Run a thin knife around the edge, release the springform, and slice with a hot, clean knife. Garnish with fresh berries or mint if you like.