Preheat your oven to 375°F (190°C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Lightly grease the paper and exposed pan.
In a medium bowl, combine the rolled oats, flour, brown sugar, salt, and cinnamon (if using). Stir with a fork to break up any sugar lumps. Pour the melted butter over the dry ingredients and mix until all the flour is moistened and the mixture forms clumps.
In another bowl, gently toss the diced strawberries with maple syrup, cornstarch, lemon juice, and vanilla extract until the strawberries are evenly coated and the cornstarch has dissolved.
Press about two-thirds (roughly 2 cups) of the oat mixture firmly and evenly into the bottom of the prepared pan to form the crust. Spread the strawberry filling evenly over the crust. Sprinkle the remaining oat mixture evenly over the strawberry layer, creating a crumbly topping.
Bake for 28-32 minutes, or until the topping is golden brown and the strawberry filling is bubbling around the edges.
Remove the pan from the oven and place it on a wire cooling rack. Let the bars cool completely in the pan, about 1-2 hours. This step is crucial for the filling to set so the bars hold their shape.
While the bars cool, make the glaze: whisk together the powdered sugar, milk, and vanilla in a small bowl until smooth. Once the bars are completely cool, use the parchment overhang to lift them out of the pan. Drizzle the glaze over the top. Slice into 9 even bars.