Prep the pan and oven. Heat the oven to 325°F (163°C). Line a 9-inch springform pan with parchment on the bottom.
Lightly spray the sides. Wrap the outside of the pan with foil to protect from any leaks if using a water bath.
Make the berry swirl. Add berries, water, sweetener, and lemon juice to a small saucepan over medium heat. Cook 5–7 minutes, stirring, until berries break down.
Stir in the cornstarch slurry and simmer 1 minute until thickened. Cool to room temperature, then blend or mash smooth. Set aside.
Build the crust. Stir graham crumbs, melted butter, sweetener, and salt until evenly moistened.
Press firmly into an even layer in the pan, going slightly up the sides. Bake 8–10 minutes, then cool while you make the filling.
Mix the filling gently. In a large bowl, beat cream cheese with sweetener on low-medium just until smooth, about 1 minute. Add Greek yogurt, lemon juice, vanilla, cornstarch, salt, and optional zest; mix on low until combined.
Add eggs and egg white one at a time, mixing on low and scraping the bowl as needed. Do not overmix; too much air causes cracks.
Assemble the cheesecake. Pour the filling over the cooled crust and smooth the top. Dollop spoonfuls of the berry sauce over the surface (you may not need it all). Use a skewer or butter knife to swirl gently, keeping the pattern on top without digging deep.
Bake low and slow. Place the springform pan in a larger roasting pan and pour hot water into the roasting pan to reach halfway up the sides of the springform (optional but helps prevent cracks).
Bake 45–55 minutes. The edges should be set while the center still jiggles slightly like Jell-O.
Cool gradually. Turn off the oven, crack the door, and let the cheesecake sit for 30 minutes. Remove from the oven, take it out of the water bath, and cool on a rack until room temperature.
Chill to set. Cover and refrigerate at least 6 hours, preferably overnight.
This step is key for a clean, creamy slice.
Serve. Run a thin knife around the edge to loosen, then release the springform. Slice with a warm, clean knife. Add a small drizzle of leftover berry sauce on plates if you like.