Preheat oven to 375 degrees and grease a 9 inch tart pan.
Combine oats, flour, baking soda, salt, and cinnamon in a small bowl.
In a larger bowl, mix the butter and both sugars together until smooth. Mix in the egg and vanilla, then stir in the flour mixture, mixing until well combined. Press dough evenly into the bottom (no need to push dough up onto the sides here) of your greased tart pan. Make sure dough is approximately the same thickness all the way across.
Bake cookie at 375 until top is puffed, golden brown, and a toothpick inserted into the center comes out clean, about 15-20 minutes. Cool completely on a wire cooling rack.
Once cookie has cooled, make the topping: use a mixer to whip the whipping cream until soft peaks form. Add the mascarpone and sugar or honey and continue mixing until just combined.
Spread mascarpone topping evenly atop cooled cookie. Arrange sliced strawberries on top and sprinkle evenly with pistachios if desired.