Preheat your oven to 350°F (177°C) and prepare a 9x13-inch baking pan by lining it with parchment paper and lightly greasing it.

In a large bowl, whisk together 2 cups of all-purpose flour, ¾ cup granulated sugar, ½ cup light brown sugar, 1 teaspoon baking powder, and a pinch of salt until well incorporated.

Cut ¾ cup of chilled, cubed unsalted butter into the dry mixture until it resembles coarse crumbs.

Reserve 1½ cups of the crumb mixture for the topping and add 1 cup of old-fashioned rolled oats to the remaining mixture.

Press the remaining mixture into the prepared pan and bake for 15 minutes or until golden brown.

In another bowl, combine 4 cups of chopped fresh strawberries, ½ cup granulated sugar, 2 tablespoons lemon juice, 2 tablespoons cornstarch, ¼ cup strawberry jam (if using), and 1 teaspoon vanilla extract.

Spread the strawberry filling evenly over the crust, then sprinkle the reserved crumb mixture on top, pressing down gently.

Bake for an additional 40-45 minutes until the topping is golden brown and the filling is bubbly.

Let cool on a wire rack for 2-3 hours before slicing.

For glaze, mix 1 cup of confectioners' sugar with 2-3 tablespoons of milk and lemon juice until smooth and drizzle over the cooled bars.