Set aside a quarter of the sliced strawberries for stirring into the finished jelly.
Stir the cornstarch and cold water together in a small mixing bowl until no lumps remain. Set this aside.
In a 2 qt sauce pan over medium heat, combine the larger quantity of strawberries, water, the remaining sugar, and orange zest and bring this mixture to a boil. Cook for 5 minutes, or until the berries start breaking up.
Once the berries have cooked for 5 minutes, use an immersion blender to puree the strawberries until they're smooth.
Strain the berries through a fine mesh sieve to remove the seeds, and rinse the pot of any seeds.
Return the strained berries to the clean pot and drizzle the cornstarch slurry into the berries, whisking constantly.
Bring the mixture to a simmer over medium heat, whisking while it heats. The berry mixture will begin to thicken after 2, or 3, minutes. When the bubbles resemble lava as they break the surface of the jelly, the jelly is done.
Remove the pot from the stove and fold in the reserved strawberries.
Transfer the jelly to a serving bowl, then set aside for topping the baked french toast later.