Preheat oven to 350F.
Melt butter in a large skillet until foamy. Sprinkle sugars evenly over butter.
Cook over medium heat without stirring until mixture bubbles all over, about 3 minutes.
Whisk well and continue to cook until it becomes golden brown, then carefully add sliced apples.
Turn heat to medium-low and cook the apples stirring now and then, until translucent, about 5 minutes. Add to raw pears and toss together, add lemon zest and juice, flour, and pinch of salt.
Pour into 8x8” buttered brownie pan, spread evenly, and sprinkle chilled oat topping over. Bake in bottom third of oven for 20-25 minutes, until topping is golden brown and fruit filling is bubbling on the edges. Serve warm with ice cream on the side.