In medium saucepan, combine berries, granulated sugar and juice. Bring to a boil stirring frequently. Meanwhile combine the cornstarch with 2 tsp of water in a small bowl. Once the berries are boiling add the cornstarch mixture and continuing stirring until thickened - only about 1 minute.
Transfer to a bowl and cover with plastic wrap directly on surface. Refrigerate about 30 minutes.
Preheat oven to 350℉. Line a 8x8 baking pan with parchment paper.
In the bowl of a stand mixer combine the rolled oats, flour, brown sugar, zest and butter. Mix on low speed until a crumbly mixture forms and the butter is incorporated.
Place ⅔ of the mixture in the baking pan and press firmly into a layer with a fork.
Spread the strawberry filling on top, then sprinkle the remaining oatmeal mixture on top pressing in lightly.
Bake 40-45 minutes until golden and bubbling.
Let cool at least 20 minutes before cutting.
Recipe by Chateau Elma at https://www.chateauelma.com/strawberry-oatmeal-squares