Start with a rimmed quarter sheet pan. Press a sheet of parchment or wax paper flat across the bottom. I never skip this step because yogurt freezes onto bare metal and is very hard to remove.
Rinse the strawberries and blueberries, then pat them very dry. Dice the strawberries into pieces about the size of a blueberry. Smaller pieces keep the bark flat and make slicing much easier later.
Pour the vanilla yogurt onto the lined pan. Use a spatula to spread it into an even layer that fills the pan. I try to keep the thickness the same everywhere so the bark freezes at the same speed.
Scatter the diced strawberries and blueberries over the yogurt. Sprinkle the granola on top next. Press everything down very gently so the toppings stay in place after freezing.
Carefully set the pan on a flat spot in your freezer. Let it freeze for about three hours, until the yogurt is completely firm. I often make it the night before, so it is ready when the kids are.
Lift the frozen sheet out using the paper. Cut it into 12 to 15 pieces with a sharp knife, or just snap it by hand. Serve it cold, and put any leftovers straight back in the freezer.