Preheat your oven to 350, then line a 9" x 9" pan with parchment paper and spray it with olive oil.
Beat butter and sugar with a mixer on medium for 2 to 3 minutes until light and fluffy.
Scrape down the sides of the bowl, then, on low speed add egg, vanilla, salt and lemon zest.
Increase speed to medium-high and beat for 2 to 3 minutes until glossy, airy and smooth.
Scrape down the sides of the bowl as needed, and when finished, add flour all at once, then mix at low speed, just until combined.
In a small bowl, toss only the strawberries with 1 teaspoon of sugar and juice from ½ a lemon.
Mash with a fork or muddler to help the berries break down. (If you want whole strawberries or larger strawberry bits, don't mash them too much.)
Now stir in blueberries and add the strawberry mixture to the blondie batter.
For a marbled effect, stir minimally with a spatula, or for a more consistently pink-colored blondie, mix until no streaks remain.
Pour batter into pan and bake for 20 to 25 minutes.
Check at 20 minutes — blondies are better slightly under baked than over baked.
The center should still be a bit jiggly when you take them out of the oven, they will continue to bake as they cool.
Remove blondies from oven and cool completely in the pan on a wire rack before slicing and serving. Enjoy!