In a small saucepan, mix together lemon juice, sugar and cornstarch. Stir in 1 cup of strawberries.
Cook over medium heat, stirring constantly. Either mash berries a bit or hit it with an immersion blender a couple of times. Cook until thick like a pudding. Remove from heat and stir in remaining chopped strawberries. Spoon into a bowl and refrigerate until ready to serve (up to a few days).
Repeat process with blueberries.
To serve: spoon into a phyllo cup, top with fresh fruit and serve with whipped cream.
Notes
I actually prefer the phyllo cups toasted up a bit. I would recommend that if you have the time. Do it ahead of time and make sure they are cooled completely before filling.