Prep the pan. Heat oven to 325°F (160°C).
Lightly spray a springform pan. Wrap the outside with two layers of foil to prevent leaks in the water bath.
Make the crust. Stir oats, almond meal, brown sugar, salt, and melted butter. Add water 1 tablespoon at a time until the mixture holds together when pressed.
Press into the bottom of the pan in an even layer. Bake 8–10 minutes until lightly golden. Cool 5 minutes.
Mix the filling base. Beat softened light cream cheese on medium speed until smooth, about 1 minute.
Add sugar and beat until creamy, scraping the bowl. Mix in Greek yogurt, vanilla, salt, and lemon juice.
Thicken gently. Sprinkle in cornstarch and mix on low until just combined. Add eggs one at a time, mixing on low after each until incorporated.
Do not overbeat.
Fill and set up the water bath. Pour batter over the crust. Place the springform pan in a roasting pan and pour hot water around it to reach halfway up the sides.
Bake low and slow. Bake 45–60 minutes, until 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, and let the cheesecake rest inside for 30 minutes.
Remove from the water bath, unwrap foil, and cool to room temperature. Chill at least 4 hours, preferably overnight.
Make the light caramel. Add sugar and water to a small saucepan. Cook over medium without stirring until the syrup turns a light amber, 5–8 minutes.
Remove from heat, carefully whisk in warm milk (it will bubble), then stir in vanilla and salt. Cool to room temperature; it will thicken slightly.
Top and slice. Spread a thin layer of caramel over the chilled cheesecake. Add a pinch of flaky salt if you like.
Slice with a warm, clean knife for neat edges.