Combine the filling ingredients in a large bowl and set aside.
Preheat your oven to 350 degrees F. Beat together your butter and sugars for at least 5 minutes, until light and fluffy. Add in the eggs, one at a time, until fully combined, scraping down the bowl as needed. Beat in the vanilla.
Whisk together your flour, cinnamon, baking powder and salt. Add to the bowl with the sugar and butter and beat until just moistened. Add in the oats and mix to combine, scraping down the bowl as needed.
Butter a 9x13-inch pan well. Press a little less than half of the dough into the bottom of the pan, then pour the fruit mixture over the top. Drop dollops of the remaining dough on top of the fruit (it doesn't have to be perfect), then sprinkle with decorative sugar (if you like - this is optional). Bake for 60 minutes or so, until golden and bubbling. Allow to cool a bit before serving warm, or cool completely, cover with foil and refrigerate for up to 2 days.