Step 1In a medium saucepan, crush 1 cup of the boysenberries with a fork. Add the water and cook over low heat just enough to soften the berries, about 2 minutes. Sieve the boysenberries through a food mill or a fine strainer to extract the seeds, collecting the juice in a bowl and then returning it to the saucepan.
Step 2In a bowl, combine the sugar (the amount of sugar will affect the sweetness of the pie), cornstarch and a pinch of salt. Slowly pour the mixture into the boysenberry juice, stirring to prevent lumps. Cook over medium-high heat, stirring constantly, just until the puree thickens, about 3 minutes.
Step 3Place a layer of strawberries in the prepared pie shell. Scatter half the remaining boysenberries on top of the strawberries, and top with just enough sauce to cover the first layer. Repeat for the second layer, adding remaining strawberries, boysenberries and sauce.
Step 4Chill in the refrigerator until set, at least 30 minutes. Serve with whipped cream.