In a bowl, mix the 200 g crushed biscuits with 100 g melted butter until the texture looks like wet sand and holds together when pressed.
Tip the mixture into an 8-inch (20 cm) springform pan. Press it down firmly and evenly using the back of a spoon or the bottom of a glass. Make sure the edges are slightly raised for a neat crust.
Place the pan in the refrigerator for at least 20–30 minutes so the base can firm up.
In a small saucepan, combine 2 tbsp cold water and 2 tsp agar agar powder. Let it sit for 5 minutes, then heat gently while stirring until it comes to a light simmer. Cook for 1–2 minutes until fully dissolved, then remove from heat and let it cool slightly (do not let it set).
In a large mixing bowl, beat 400 g softened cream cheese, 120 g powdered sugar, and 1 tsp vanilla extract until completely smooth and creamy with no lumps.
In a separate bowl, whip 200 ml cold whipping cream until soft to medium peaks form. It should be fluffy but not stiff.
Gently fold the whipped cream into the cream cheese mixture in batches, keeping the mixture light and airy.
Stir the slightly cooled agar mixture into the cheesecake filling and mix quickly but gently to combine evenly.
Fold in 150 g finely chopped fresh strawberries, distributing them throughout the filling.
Pour the cheesecake mixture over the chilled crust and smooth the top with a spatula. Tap the pan lightly on the counter to remove air bubbles.
Place the cheesecake in the refrigerator and chill for at least 4 hours, or until fully set and firm to the touch.
For the topping, add 200 g chopped strawberries, 60 g sugar, and 1 tbsp lemon juice to a small saucepan. Cook over medium heat until the strawberries release juice and soften.
In a small bowl, mix 1 tsp cornstarch with 2 tbsp water to make a slurry. Stir this into the strawberry mixture and cook for another 1–2 minutes until thick and glossy.
Remove the topping from heat and let it cool completely to room temperature. It will thicken more as it cools.
Once the cheesecake is fully set, gently spread the cooled strawberry topping over the surface in an even layer.
Arrange the halved fresh strawberries around the edge or on top as desired.
Chill the cheesecake for another 30–60 minutes to let the topping set before slicing.