Line a 9 inch square baking pan with parchment paper with long edges. This is optional but will help you lift out the blondies for easy glazing and slicing later.
Cream the butter and sugar until fluffy. Beat in the egg.
When the egg is fully incorporated, beat in the lemon juice. The lemon juice will probably not completely incorporate at this point, that's ok.
Whisk together the flour, baking powder, and salt, and add to the wet ingredients, mixing until no more dry flour is present.
Gently fold in the diced strawberries. Spread the mixture into your pan and spread out as evenly as possible. The dough is thick, so take your time. I like to use a large offset spatula for this job.
Bake for 30-35 minutes, just until starting to turn golden around the edges and the center is set. A toothpick inserted in the center will be moist but not wet. Don't over-bake these, or they will dry out. Let cool on a rack.
Trim the strawberries for the glaze and puree in a small food processor. Strain the puree through a small strainer to get 1 Tbsp of puree. Whisk together the sugar, puree, and lemon juice until no lumps remain. If your glaze is too thin, add a touch more sugar. If it's too thick, add more lemon juice.
Spread the glaze over the cooled blondies. Let the glaze set before slicing.
Video
Notes
When you measure out the flour, first fluff the flour, then scoop it with your cup, and then level it without compacting it. This method helps insure an accurate amount.
If you don't want to bother with the strawberry puree for the icing, just use lemon juice. I don't recommend using strawberry jam because it would be too sweet combined with the sugar.
Do a little test before glazing your blondies. Spoon a bit of glaze on the blondies and see how it does...if it soaks right in it is too thin you can add more sugar. If it's too thick you can thin it down with more puree or lemon juice.
For a variation, try this with diced rhubarb, blueberries, or raspberries. You could also use a combination of strawberries and rhubarb.