Prepare a square 8x8 baking pan with parchment paper on all sides, then preheat the oven to 350F/180C.
In a small bowl, combine the diced strawberries and lemon juice and set them aside.
In a medium bowl, whisk together the room temperature melted butter, lemon zest, and light brown sugar until combined.
Whisk in the egg and egg yolk until combined.
Use a rubber spatula to fold in the flour, baking powder, salt, and strawberries until just combined. Then evenly spread the blondie batter into the prepared pan, using a spatula to smooth the top. Press a few thinly sliced strawberries on top.
Bake the blondies for 20-30 minutes, or until a toothpick inserted into the middle comes out with just a few moist crumbs.
Let the pan cool completely on a wire rack, then cut into 20 bars.